The 20x25 filter ideally is better, but the 16X25 filter is OK for your 3 ton AC. I would not make a big deal out of it. This is the configuration I have and it has worked well for me. A Merv 8 filter would be less restrictive, but you can use the Merv 11 if you want better filtering.

Thanks Mike- how long have you been in ac? Very knowledgeable ...we need some of that knowledge here in Texas .. Thank you for all your time and advice very much appreciated.