Anupam, unless you need to cut light output there's no need for diffusers. When the flash is close to the subject it is effectively a large, not a point, source.

Yes, if you want to light the background putting a third flash on a tripod is a good idea. Just use GN arithmetic with an adjustment for magnification. You may think you don't need more gear, but to get full flexibility you'll want a manual flash with power level control. The Vivitar 283 with VP-1 is ideal for this. Out of production, very available used. And since you have a flash meter testing to find out what the flash actually does will be inexpensive and easy.

Any old slave trigger will do. Except the ones sold for triggering flashes from some P&S cameras' built-in flash; these little beasties have the photocell's eye at 90 degrees to the flash's axis. I have three, all Konica, and they're pretty useless. I've been using cube-shaped slaves that look like the Kaiser slave that B&H sells. Mine are now 30+ years old, haven't had a problem yet.

Its better to use tiny fixed-output flashes close to the subject than to use somewhat more powerful auto-TTL flashes. Flash output can be adjusted with ND gels. And in this case incident metering gives more consistent results than does metering reflected. Calibrating a flash rig as I've told you how to do is equivalent to metering incident.