no, what I would advice if your batteries can't start the engine
- put battery switch to "off"
- use jumper cables or better yet normal cables with lugs between the spare battery and the starter on the engine
- start engine
- put switch to both
- remove spare battery
If you connect the spare battery to the other two (which are empty) you discharge the spare battery into the empty ones. Better use it to start the engine.
After starting engine, reconnect the normal batteries before removing the spare battery. If you remove the spare first, you can blow out diodes in the alternator
(it must always see a battery).