Deck Time towards 3rd AE!

Can someone explain how the 90 days of deck service on vessels over 100 GT can apply to a 3rd AE license. Is that exactly what it states? For example let’s say I have my time in the engine room as a QMED (720) but I dont have the total engine room sea service days (1080) but I have AB time on deck. I can apply that (90 days) towards the 3rd AE licence?

Maybe because deckhands on little boats might help with engine duties or be tasked with routine engine tasks?

Because the CFR says so. See 46 CFR 11.516(b).

Yeah, per the CFR, it looks like you can.