I'm upgrading my PC this winter and the cheapest and most effective upgrade I could do to my GPU setup was to go from 1 GTX 760 to 3. I was thinking 2 but I have a larger budget then I thought. Is having three graphics cards complicated. What kind of cooling and power supply would I need? Is an NZXT Phantom 410 mid tower case enough to house the cards. Getting 2 more 760's cost around 520-540 dollars while getting one 780 or 780ti costs around 700+ and if it works I get great performance.
You should probably do some research in to the games that you play and their scaling when using tri-sli. There are many games where it doesn't scale well, or at all. You also need a pretty high speed processor, or lots of pixels that need to be fed in order for it to be really useful. If you are just running a single 1080p or even somewhat a 1440p, you might be better served to sell the one 760 you have and switch up to xfire 290, 290x's or 780, 780ti's (or even just sli 760's or a single beefy card)
For about $740 (selling a 760 as well) you can get a pair of 290s which would scale much better, and give better performance. 3 760s is a little worse than 2-way 780 SLI, and an R9 290 is similar to a 780.
Mid-tower cases aren't really built for 3/4-way SLI, and you'll most likely need a different motherboard to even support it. You're also going to be limited going forward having only 2GB of VRAM. 3 way SLI just isn't a very good option unless you have top of the line cards.