Title of 80's animated childrens film

Hi, I've been searching everywhere for the name of this childrens film but can't find it.

The basics of it are... there are 3 air planes (red blue and yellow), and they connect together and create a robot. They try and attack a fortress in the sea but get captured and gets it robot arm cut off.

The box for the video tape had a sort of honeycomb pattern design on the background.

I know its a bit vague, but its all I can remember!

If anyone knows what might be, please let me know. You can email me on jon@jonrichford.co.uk