It depends on the regen rate - if it's the same, then it stacks.. for example if you have/wear slow regen + slow regen=faster regen. Slow regen + fast regen= fast regen. Fast regen + fast regen= very fast regen.
Sadly it's quite some time since i played BG2 and I don't remember the regen rate of the items .. I just remembered that regen behaviour in general, because around the time when BG2 was released I participated in a discussion about this topic and because I always liked life-regen (no matter how small). About the "triple" combo.. I can't say.. all I remember is, that there were not that much regen items anyway, so I guess I never tried that because of the lack of similar-rate items.
Ring of Regen x2, Fong Wei's Ioun Stone, Axe of Unyielding +3, 25 con = 1 hp/ round each. Total of 5 hp/round (but is still overwritten, since Gaxx and AotU+5 are singularly better)
Staff of the high Forest = 1 hp/ 2 rounds
24 Con = 1 hp/ ~3 rounds
Pearly White Ioun stone = 2 hp/turn
23 Con = 1 hp/ 5 rounds
22 Con = 1 hp / ~7 rounds
21 Con = 1 hp/ ~9 rounds
20 Con, Dragonslayer = 1 hp/ turn, 2 hp/turn total
There are also some other minor benefits of having Con higher than 16 for non-Warrior classes e.g. Fatigue bonus (which influences how long you can go without resting before you become fatigued) goes from (I think) -4 at 14-17 to -5 at 18 Con and, for what's it worth, you can drink more alcohol before becoming intoxicated with higher Con.
Also, if you're interested in using the Claw of Kazgaroth, which reduces Con by 2, but gives benefits to most saving throws and an AC boost, then 18 Con would give you headspace to have this reduced back to 16.
Am not sure if any of the above make it worth putting more than 16 into Con for a Monk, as there are probably other stats that would benefit more, but if you have them spare, going to 18 Con is not necessarily a total waste.
Also, although they can't be Monks, Gnomes, Dwarves and Halflings all benefit from Constitution-based saving throw bonuses, 14-17 Con gives +4 to saves, 18+ gives +5 to saves, which arguably *is* worth trying to get.
Comments
Ring of Gaxx = 1 hp/3 seconds (2 hp/round)
Ring of Regen x2, Fong Wei's Ioun Stone, Axe of Unyielding +3, 25 con = 1 hp/ round each. Total of 5 hp/round (but is still overwritten, since Gaxx and AotU+5 are singularly better)
Staff of the high Forest = 1 hp/ 2 rounds
24 Con = 1 hp/ ~3 rounds
Pearly White Ioun stone = 2 hp/turn
23 Con = 1 hp/ 5 rounds
22 Con = 1 hp / ~7 rounds
21 Con = 1 hp/ ~9 rounds
20 Con, Dragonslayer = 1 hp/ turn, 2 hp/turn total
Also, if you're interested in using the Claw of Kazgaroth, which reduces Con by 2, but gives benefits to most saving throws and an AC boost, then 18 Con would give you headspace to have this reduced back to 16.
Am not sure if any of the above make it worth putting more than 16 into Con for a Monk, as there are probably other stats that would benefit more, but if you have them spare, going to 18 Con is not necessarily a total waste.
Also, although they can't be Monks, Gnomes, Dwarves and Halflings all benefit from Constitution-based saving throw bonuses, 14-17 Con gives +4 to saves, 18+ gives +5 to saves, which arguably *is* worth trying to get.