The wiki citation of +3 is, indeed, completely unverified, as is the claim that Signboard and Map give +2. Heck, I'm not sure that anyone even confirmed that the Map alone gives +1, though I could be wrong on that account. For other crafts, it's been confirmed that the trio of furnishings give +5 (thus the new tier 1 rate on Dusk Gloves).
I've got 110k points right now, working up toward the Map. 200k after that and I'll have the trio. If I can get some specific data on the levels of fish above 100, I can do some testing. I'll see what I can do to find such fish.
Also, for the reference, the Stall alone gives Mega Moglification: Fishing. It allows the fish to be more quickly discerned, which reduces the amount of time a fish takes to bite, essentially.