How can I make Intel Core i7 Mac perform at i3 or i5 level? I need to do some performance testing, and I need the MacBook to behave at i3 or i5 speeds. Is this possible? If yes, how?

I have the MacBook Pro Retina / macOS Sierra 10.12.6 / 2.2GHz Intel Core i7.
It's not 100% what you're looking for (in that the controls are not fine grained enough to say "make my i7 4770HQ perform like an i3 4150"), but something that will tweak your CPU's TurboBoost settings can help.

One application that can do so is Turbo Boost Switcher - it may be worth giving it a try. Another way is to disable CPU cores using Instruments.app (part of XCode). You could try a combination of the two to slow down your machine, perhaps benchmarking as you go (eg, using Geekbench or similar) until you reach the desired level of performance.