High Quality AGM's can accept up to their amp hour rating as a charge rate...lesser AGMs will at least allow 50% of capacity charge rates. In any event, since you have about 450 amp hours total in 3 x 4D's..... getting down 50% of capacity will create a charging
need for 225 amp hours...with your 120amps of alternator power, you will be able to put all but the last 10% of that back (call it 200 amphours) in less than 2 hours of run time.
Now lets look at wet cells as an alternative..you can put back in about 20% of rated capacity during bulk charging
which means about 90 amps per hour. So in 2 hours you will have put back about 180 amps...very close to what you can do with the AGM's...
So...in your boat, with your charging system, AGM's will not appreciably reduce charging time and at 2-3 times the price will not have an economic payback.
Your decision to invest in AGM's will need to be based on other factors such as no maintenance
, spill proof, doesn't lose much charge when not in use. Otherwise stick with decent flooded cells.
To be clear...I am speaking about YOUR situation. I have AGM's and love them AND can justify them economically in MY situation!