Can a coin cell with rating >= 500 mAh be used to drive a set of 3 IR LEDs if each takes 100 mA forward current? I'll be drawing 300 mA (3 LEDs) current continuously from the cell, I heard that coil cell can deliver only low discharge current because if high current is drawn voltage drops very fast to thresh hold voltage of 2V.
The internal resistance of a 2450 coin cell is about 10 Ω when fresh (see https://data.energizer.com/pdfs/cr2450.pdf). If you consumed 300 mA, the voltage would drop by 3 V -- and you wouldn't be able to drive the LEDs.
These cells are not suitable for high continuous currents; they are intended for very low continuous drain (at les than 1 mA), or brief high current pulses.
Although these are rated at over 500 mA.h, this does not mean you can consume at a load of 500 mA for 1 hour -- but you can consume at 1 mA for 500 hours.
Absolutely not. Coin cells can give only few milliamps at a time in the circuits they're normally used, I tried them in my project that pulled like 50-70ma (I was young), don't remember exactly, but it dropped immediately from 3.2 down to 2.65 when connected. It is indeed used for ultra low-power applications where current draw is sub-milliamp. Li-ion 3.7V batteries are a lot better for this application, but you still need some serious capacity if your LEDs are going to be ON for any signigicant amount of time and not one second per hour of work. So if some 500-1000mah battery physically fits in your project, you better use that, they don't cost much anyway and they're also rechargeable! I think it would be a win-win.