That's Pachanoi, and the general concensus is that the mescaline content varies from almost nothing to very potent in that type cacti. The good news is that it's Mescaline and, while you can get in over your head on any psychedelic drug Mescaline is relatively gentle and forgiving in that regard, meaning if you get a little extra it's usually a bonus. Odds are that Pach isn't likely to be one of the few extremely potent varieties.
So the only way to tell is to try it. Assuming those cacti are rooted I'd cut all three back and brew them up, leaving the stumps to grow back. Divide the tea into three doses and drink each separately with about 2 hours between the first dose and second, and then the third at about hour 6 if you feel you need it.
I would start in the morning because it's pretty much an all day deal.
Enjoy.
My flesh moves, like liquid. My mind is cut loose.