Split html table, odd columns to even

Split html table, odd columns to even

I would like a single <table> tag to render like this:
+_____+_____+_____+
| | | |
+_____+_____+_____+
| | | |
+_____+_____+_____+
| |
+________+________+
| | |
+________+________+
| | |
+________+________+
Possible? I am lost on this one. I've been tableless for a long time.
Documentation on changing the column numbers half way through is not
clear. I know it could be done with col-span if the shift in columns was
even, but how to go from odd to even?