2 to 3 dry bags
I’ve only done a few 3 days camping trips, so I’m not an expert.
“3) 1 bag for tent, thermarest and cookset”
I don’t think tent needs to go in a dry bag. 1) bottom of tent is already waterproof, that’s the part on the outside when my tent is packed. 2) Even if a bit of water gets IN the tent, you can air it out pretty quickly once you wipe away the water droplets.
Same goes for Thermarest.
Cookset REALLY don’t need to be in a dry bag. However, I carry at least two match books, each in their own ziplock bag.
Main thing is keep the sleeping bag bone dry, and some dry clothes to change into. Unless you actually have a flooded hatch, double garbage bag will keep most other stuff reasonably dry.
The only time I worry about keeping stuff VERY dry is if I ever do spring/fall camping trip in the north, where the weather can be damp and cold. There’s no chance to dry out anything once they got wet. I would put the thermarest or even the tent in the dry bag in THAT kind of conditions. The rest of the year, 2 medium size dry bag (1 for sleeping bag, 1 for clothing) and a small one for misc stuff are all I need.