I'm the type to get easily chilly. At 75º I would be comfortable in my 5mm full suit, and would get a bit chilly in a 3mm. But, that said, I did do a number of dives at 75º in my 3mm when that was all I had. It was almost essential for me to limit it to a two-dive day in that. A hood and gloves helped. A warm day/surface interval made a difference too. At 82º I would be comfortable in my 3mm full suit.
My usual dive buddy is the opposite and is usually warm. I think he would be fine in a 3mm at 82º and might even be able to wear less. At 75º I think he would still be comfortable in the 3mm.
So...basing it on my experience if you are a chillier type person, you might be able to get by with just a 3mm full suit, if you supplement it with a hooded vest and gloves for 75º. Especially if you are doing 2 or fewer dives per day and it's not really cold/windy on the surface.
But.... it's so variable from person to person. I've read where other people are comfortable in a 3mm shorty in 6xº water, for example. I would be an ice cube!
I have a Waterproof brand westuit and I really like it (actually I have three of them now, 3mm, 5mm and a newly purchased 7mm.... got too chilly at 69-70º in my 5mm, even with a hood and the Waterproof 7mm was a real bargain on closeout).
I like the Waterproof suits because they have *tons* of sizes and I don't usually fit into standard ones. Also they have nice features such as double cuffs and ankles, with zippers (one cuff goes inside the booties, one outside, etc.), a short neck zipper in front that you can undo on the surface to get a bit more breathing room, etc.
If you can handle the "Power Ranger" look, and if they have one in your size, Waterproof's "last year" 3mm is on closeout now for the bargain price of $99.
Womens' size chart, to give you the idea:
Link to their close-outs:
Waterproof - CLOSEOUTS | Waterproof Facing Reality
