My application is a 36v golf cart (3 x 12v) - I'm assuming that I could use this for a battery maintainer (one "channel" for ea 12v battery)?

Yes. Channel 1 to battery 1, channel 2 to battery 2, channel 3 to battery 3.

I have 2 group 31 Deep cycle batteries in series. Is the charge time with this charger longer leaving them connected? what's the difference?

As long as there is nothing hot feeding off the battery should charge just as fast either way. Leaving the charger connected all the time won’t hurt your batteries. It will go into standby on its own once the batteries reach full charge

ASIN	B08G3F4RN8
Best Sellers Rank	#2,906 in Automotive ( See Top 100 in Automotive ) #28 in Battery Chargers
Customer Reviews	4.5 4.5 out of 5 stars (3,376)
Date First Available	August 17, 2020
Department	Unisex-Adult
Item Weight	6.09 pounds
Item model number	GEN5X3
Manufacturer	NOCO
Product Dimensions	2.2 x 7.9 x 6.5 inches

Genius GEN5X3, 3-Bank, 15A (5A/Bank) Smart Marine Battery Charger, 12V Waterproof Onboard Boat Charger, Battery Maintainer and Desulfator for AGM, Lithium (LiFePO4) and Deep-Cycle Batteries

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