I use nulon pre-mix,buy at repco.

Drain a little out and compare to fresh coolant.There's a coolant change diy on here by aaronng,I can't find it,can someone?

It has an old vs new comparison pic.

3 years is heaps.Swap it out for 20 bucks..

Undo clamp on bottom hose on radiator and stick the garden hose in the top hose and flush the ols out completely (remove thermostat).

Or pay $140 to have it reversed flushed.Only needs doing if old though.