ron2456 :
1060 3gb is 60% - 70% performance of 6gb.
This is not very accurate. In nearly all current games at 1080p, the 1060 3GB offers around 90% of the performance of a 1060 6GB, and the difference can be even less than that in situations where performance is limited by the CPU. This is due to the 3GB card having 10% of its cores disabled, in addition to having less VRAM. The lower amount of VRAM can potentially make a difference as well, but most of today's games can get along fine at high settings on 3GB. There's the possibility of it making more of a difference in future games that utilize more VRAM, where the card might lose more performance when it needs to access system RAM, but turning a setting like textures down a bit should likely keep a game's VRAM use in check. Considering the 3GB card has a roughly 20% lower MSRP, it's arguably a reasonable trade off in my opinion, and I would consider the 1060 3GB to be at least a better value than a 1050 Ti, which is significantly slower.
The only argument I'd have against buying a 1060 3GB now is the same argument against buying the 6GB version, or most other cards at the moment. These cards are nearly two years old at this point, and yet they are currently priced higher than what you could get them for back in 2016, due in part to some shortages a number of months back. Nvidia should be launching a new generation of graphics cards over the coming months, and there's a strong possibility that we will be seeing GTX 1070-level performance around this price range later this year. The 1060 3GB launched for US $199 in 2016, and the 6GB version for $249, so it's difficult to consider it worth spending more than that at this point, especially with new, faster cards likely being just around the corner.
As for the cooler, a GTX 1060 is about a 120 watt card, so it could technically get away with a single-fan cooler, though a dual-fan cooler will almost certainly be able to run cooler and quieter. Personally, I would look for something with two fans in this power range, though as far as performance goes, a single fan should be able to get the job done.