Bootstrap buttons/input fields on same line

Any of those help You can use following style and if you want bigger then increase padding.

input, button{ 
    padding: 7px;    
}

will help you maybe you are trying to accomplish this http://silviomoreto.github.io/bootstrap-select/ there have a lot of options that you can use, like select more that 1 in a dropdown

hop of those help? You'll need to make the column width wider. Since you're using col-lg-3 class, the last button is wrapping to the next line. Try changing it to a larger column (i.e. col-lg-6). To answer the second part of your question, you can add a CSS rule like .btn {margin: 0 5px;} if you want some space between the buttons.

With these it helps First, each form-group automatically gets its own line. It employs a clearfix to ensure that nothing else wraps around it. Second, even if it didn't, each of your label/field combos are consuming 12 columns. Since there's only 12 columns to work with in the Bootstrap grid, each would wrap to a new line anyways.
There's two options here. If you want multiple fields on the same line, under the same label, you can simply put them all within your col-md-10 div within the form-group:

<div class="form-group">
    <label class="control-label col-md-2" for="@Html.IdFor(m => m.FirstName)">Name</label>
    <div class="col-md-10">
        <div class="row">
            <div class="col-md-4">
                @Html.EditorFor(model => model.FirstName, new { htmlAttributes = new { @class = "form-control" } })
                @Html.ValidationMessageFor(model => model.FirstName, "", new { @class = "text-danger" })
            </div>
            <div class="col-md-4">
                @Html.EditorFor(model => model.MiddleName, new { htmlAttributes = new { @class = "form-control" } })
                @Html.ValidationMessageFor(model => model.MiddleName, "", new { @class = "text-danger" })
            </div>
            <div class="col-md-4">
                @Html.EditorFor(model => model.LastName, new { htmlAttributes = new { @class = "form-control" } })
                @Html.ValidationMessageFor(model => model.LastName, "", new { @class = "text-danger" })
            </div>
        </div>
    </div>
</div>

<div class="row">
    <div class="col-md-4">
        <div class="form-group">
            @Html.LabelFor(model => model.FirstName, htmlAttributes: new { @class = "control-label col-md-2" })
            <div class="col-md-10">
                @Html.EditorFor(model => model.FirstName, new { htmlAttributes = new { @class = "form-control" } })
                @Html.ValidationMessageFor(model => model.FirstName, "", new { @class = "text-danger" })
            </div>
        </div>
    </div>

    <div class="col-md-4">
        <div class="form-group">
            @Html.LabelFor(model => model.MiddleName, htmlAttributes: new { @class = "control-label col-md-2" })
            <div class="col-md-10">
                @Html.EditorFor(model => model.MiddleName, new { htmlAttributes = new { @class = "form-control" } })
                @Html.ValidationMessageFor(model => model.MiddleName, "", new { @class = "text-danger" })
            </div>
        </div>
    </div>

    <div class="col-md-4">
        <div class="form-group">
            @Html.LabelFor(model => model.LastName, htmlAttributes: new { @class = "control-label col-md-2" })
            <div class="col-md-10">
                @Html.EditorFor(model => model.LastName, new { htmlAttributes = new { @class = "form-control" } })
                @Html.ValidationMessageFor(model => model.LastName, "", new { @class = "text-danger" })
            </div>
        </div> 
    </div>
</div>

I wish this help you
EDIT: I might be approaching this problem the wrong way.. Now im thinking maybe i should find the button that was clicked and then get the previous element's value

<div class=verktyg>
  <form action='sell_items.php' method='POST' id='ajax'>
    <table class="inventory">
      <tr>
        <td class='verktyg-first-td'>Item 1</td>
        <td>10</td>
        <td>
          <input type='number' name='item_1' value='10'>
        </td>
        <td>
          <button type='submit' class='submit_button'>sell</button>
        </td>
      </tr>
      <tr>
        <td class='verktyg-first-td'>Item 2</td>
        <td>20</td>
        <td>
          <input type='number' name='item_2' value='20'>
        </td>
        <td>
          <button type='submit' class='submit_button'>sell</button>
        </td>
      </tr>
    </table>
  </form>
</div>

 // prevent for submission
 $("form").submit(function(e) {
   e.preventDefault();
 });

 // when submit button is clicked
 $(".submit_button").on("click", function() {
   self = $(this);
   form = self.closest("form");
   url = form.attr('action');
   type = form.attr('method');
   parentTr = self.parent("td").closest("tr"); // get the parent <tr>
   data = {};

   inputElm = parentTr.find("input[type=number]"); // find input element closest to the clicked button
   name = inputElm.attr('name'); // get input name
   value = inputElm.val(); // get input val
   data[name] = value;

   // send ajax request
   $.ajax({
     url: url,
     type: type,
     data: data,
     success: function(response) {
       console.log(response);
     }
   });
 });

    <td>
      <input type='number' name='item_1' value='10'>
      <button type='submit' class='submit_button'>sell</button>
    </td>

<button type='submit' class='submit_button' name="item" value='button_value'>sell</button>

// get button val
value = self.val();
name = self.attr('name');
data[name] = value;