With an A-rated distro (no engineering), it can run two small mining lasers for about 2 minutes. Not sure if heat would be a problem towards the end or not, though.
Two small mining lasers is 3 MW of drain. 3A PD has 2.8 MW of WEP recharge at 4 pips. The total drain is thus .2 MW. 3A PD has 24 MJ of storage. MW = MJ/s, so it would take 120 seconds to drain 24 MJ at a total drain of .2 MW.
I don't recall having any heat issues when I used it and I did a lot of mining in it. It made a decent little miner.
The only issue I had was staying awake. Two small mining lasers is just soooo slow and 500t is just sooo much for an engineer.
Not to mention all the work it took afterwards to clean her up so that she would be respectable enough to carry luxury passengers again..