Loop 3 posts column wise and continue the loop into same columns

Question

If that col-4 class does what your example suggests it does, then just wrapping each post in col-4 would achieve the same effect:

<div class="col-4">
Post 1
</div>
<div class="col-4">
Post 2
</div>
<div class="col-4">
Post 3
</div>
<div class="col-4">
Post 4
</div>
<div class="col-4">
Post 5
</div>
<div class="col-4">
Post 6
</div>

Then you don’t need to do anything special with WP_Query at all.