[Dave M]

I have a 36' dock. Have cross piece at end and 2nd cross piece ~8' from that.
In your case, one at end, one at the 3rd section. Also my dock sits on the floor. Add aluminum footings to end of piping, saves hammering. Dock is very rigid even with boat attached.
If putting cross pieces make sure you use couplings at middle of pipe that goes side to side and at the vertical pipes. If you have to raise/lower deck one of the sections of each cross brace will have to be taken apart.
If you have galv piping to hold your section up get a mini come-along to raise
and lower dock. Do not have to get in the water except the cross pieces.

Dave M

PS I do live in a cove so wave action is not as severe.

Forgot to mention. My dock is 6' wide. Width plays into this as well.