i have an older model NRS raft, the Otter 11.5 (the model prior to the current Otter 120). my 11.5 model would be very, very tight for fishing 3 people (front seat, stern seat, and captain chair). my current fishing frame only has the elevated front swivel seat, and the raft is tight for two of us with a day's worth of gear and a softside cooler. i don't think the additional space in a 12' boat at more/less the same width (the 11.5 and 120 both use a 48" NRS frame) would give enough extra room to comfortably accommodate 3 people.
if your desire is to fish a crew of 3 (2 fishing, 1 rowing), i would suggest a 13' raft as your minimum size. i think the NRS Otter series is a good choice at a more reasonable price for recreational use, but it seems they do not hold up to commercial abuse well based on posts from other users. i now own two Otters, the 2004 11.5 for fishing and play/paddle boating and a new 2009 Otter 150 for big water and overnights. just my 2c, later.
if you run a smaller frame, the risk is it will slide into the boat if it should move or shift in a rapid, or if a strap loosened. the frame side rail should sit at (or slightly outside of) the mid-point of the main tubes. most 13' boats are around 6 to 6.5' wide with 18 or 19" tubes.
as a specific example, the NRS 13' boats are 6' 2" wide with 18" tubes (74" - 18" = 56") and NRS recommends a 60" frame width for these boats.
when you say "3 fisherpeople" do you mean 3 people holding rods along with the oarsman, for a total of 4 people in the boat? most fishing frames are set up with 2 fishing seats only, and adding a 3rd would be nearly impossible if you are fly fishing. could be a little easier with two people up front spin fishing, and sharing a flat seat or cooler to sit on.
if you are hoping to fish from the captain's chair, i would advise against it unless you are literally in lake flat water. when i am rowing with a person fishing up front, i am constantly adjusting boat position due to current shifts and eddy lines to put them on the most productive water. if i drop an oar to try to fish, it seems like we are out of position or into the bank fairly quick even on slow moving water. i just let them fish while i stay properly intoxicated, and switch turns in the fishing chair after a few fish each turn. just my experience on the rivers i fish, could be different for others with better skills or less beers.
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