I have played with this problem:

And have solved it this way:

section 1 & 3 are DIVs with buttons
section 2 is blank DIV
be care about the GROW value
1: 1
2: 20
3: 0 <- this is important - if you give there 1 (or more) ou will have button with right blank margin that canNOT be handled later with margin-right: 0px
<!-- buttons for CRUD table -->
<div id="toolbar" style="display: flex">
<div style="flex-grow: 1">
<a href="javascript:void(0)" class="easyui-linkbutton" iconCls="icon-add" plain="true" onclick="newItem()">New Item</a>
<a href="javascript:void(0)" class="easyui-linkbutton" iconCls="icon-edit" plain="true" onclick="editItem()">Edit Item</a>
<a href="javascript:void(0)" class="easyui-linkbutton" iconCls="icon-remove" plain="true" onclick="destroyItem()">Remove Item</a>
</div>
<div style="flex-grow: 20">
</div>
<div style="flex-grow: 0">
<a href="javascript:void(0)" class="easyui-linkbutton" iconCls="icon-reload" plain="true" onclick="tableReload()">Reload Table</a>
</div>
</div>
REM:
DIV id="toolbar" - has
style="display: flex" - also all his childrens have this also (not need to define style section in css)