I have an intel z170 chipset and a gtx 1060 6gb gddr5 and im wondering if i can buy and gtx 1060 3gb and that will still work and not slow my computer dow.

3 GB GDDR5 RAM is not much today and it gets worse in the Future. run your Favorite Games with GPU-Z  open in the Background. then go to the Sensors Tab and search for Memory Usage. klick on the Settings and schoos Maximum. it will show you how much VRAM you need for your Games to run without Issues.

but why would you want to do that in a first Place?

Sounds like maybe you have an Aurora-R5, and are wanting to increase your graphics power by adding another GTX-1060 to SLI.

1. If you are going to upgrade to SLI, the graphics cards should match very closely. I'm pretty sure this includes on-board memory.

2. The Aurora-R5 should have the optional 850w Power-Supply for SLI use.

3. GTX-1060 is kinda low-end. Adding another will only add about 50% from second card (machine won't be twice as fast graphically). If you have 850w PS, a better move would be to install a single GTX-1070, 1080, etc.). IMO, a single-card solution works better in Aurora R5/R6, plus you don't have to mess with SLI.

1060s have no SLI Connectors.