why does my gas tank have water after being parked for 3 or more days

Simple condensation will collect in tank, especially if where you live is humid and has a big difference in the high an low temperatures ..day and night. As in 90° day/60° night. But geez, that much water almost seems like somebody is messing with you, putting water in. Possible...? Maybe...? If you can rule that out, condensation, as stated, will collect but that is too much for ordinary conditions. Best suggestion is fill it up, and keep it full...even if you only drive 50 miles or so, keep it topped off and see what happens then. Also, an OBDII scan wouldn't hurt, CEL lit or not. The EVAP may have an issue.

And clean water or coolant....? Afterthought, going under assumption we are talking just water. But if it's engine coolant, whole 'nother ballgame

condensation is one thing.. first change fuel filters...union oil some years back had some bad gas sent to some of its dealers...spent 2 months dropping tanks...anyway did yo get a bad batch? buy some of this...http://www.autozone.com/fuel-and-engine-cleaners-additives/fuel- additive-and-cleaner/valvoline-12-oz-water-remover/33412_0_0/

It is likely he has E-10 in it and it got polluted with enough water to phase separate the fuel. Tank needs to be completely drained and fresh gasoline installed. No amount of additive will fix phase separation. Keep the tank at lease half full and don't worry about the condensation unless it is on the inside of the filler neck or tank.