i assume they're all fighting each other at the same time? this is actually a really hard choice, since all of them pretty much counter each other and are more than able to take each other out. but seeing how they cannot use tm/hms and all that, i'd have to give it to blastoise most of the time. neither charizard nor venusaur learns sunny day through leveling up (which would literally f*** blastoise up) and to top it all, blastoise actually learns rain dance which will boost his water attacks.

charizard has a chance to ohko venusaur with a flare blitz (charizard being the fastest, he'd attack first) and even in the rain there's a chance charizard can knock venusaur out with two flare blitz.
a hydro pump in the rain will one-shot charizard. blastoise can probably survive some of venusaur's attacks by boosting its defense, using either withdraw, skull bash (which also attacks after raising defense) and iron defense (which is better than withdraw), or he could just stall venusaur using protect. not to mention, blastoise can also learn flash cannon, which will take away roughly 20% of venusaur's hp.

i'd imagine it would go something like charizard one-shots venusaur and blastoise then proceeds to take out charizard. or charizard would attack venusaur and blastoise would set up rain, stall and/or raise defense, while he does that charizard finishes venusaur off and blastoise then takes out charizard.

just something quick i wrote, both charizard and venusaur are capable of winning under different circumstances. venusaur sleep powdering blastoise and takes him out, then charizard takes out venusaur or venusaur sleep powders both, takes out blastoise and then proceeds to synthesis, double edge, leech seed the hell out of charizard. however this only works if venusaur survives the first flare blitz from charizard (which he probably will, flare blitz only has a 12.5% of one-shotting venusaur @ no ev and neutral nature) and if he's able to actually hit them with sleep powder (s***ty accuracy).

but in the end, i still see blastoise winning most of the times.