btk,
Here's what you do. Fire up the EValuator tool here...
http://www.tworags.com/index.php?ACTION=odds
Hard to say what his hand range is without knowing any of the action to this point, but I think it's safe to assume he's got one of the following hands given his willingness to push that much dough in your face:
TPTK...say Ad Ks
a set...maybe JJ
a made straight...maybe KT
2 pair...maybe AQ
a lower flush draw...T9s
or a bluff
Of the bunch, you're only in good shape against the last two, but based on the money in pot, it still might make sense to call that big bet since there is a shot you spike a T to make a straight or a spade to make the nut flush.
To use the EValuator, input the amount in the pot (5000), the amount of your call (3000), your hand KQs, and the board, in this case, As Qh Jc 5s.
Then input five of the above hands and a guess at the probability he's holding it. I used...
Ad Ks...20%
Jd Jh...20%
Kd Th...20%
Ad Qd...20%
Ts 9s...20%
You might feel the percentages should be different based on the betting thus far; I chose these for the sake of example. In my experience, you could see someone push with any of these hands.
Then hit the CALC button. It'll return a calculation of the net expected value; if it's positive, you should call. If it's negative, you should lay the hand down. Given the info above, it's positive $36 in this case, but that's not a lot based on the size of the pot and it will vary depending on the percentages you apply.
If you'd like to re-run the calculation with different hands or probabilities, click the Run Again button. Play around with a couple of scenarios, but think back on the pre-flop, flop and turn action to narrow the hands down.
To answer you're question, I'd probably lay the hand down unless I had a great read on the pusher as a guy who could push a flush draw with one card to come. They're out there, but the guys who can push a set, a made straight or two pair to scare off a big flush draw are more plentiful. Note that if you reduce the probability you put on the T9s hand to 10% and increase one of the others, your EV drops about 500 bucks. I say save the chips and wait for a better spot.
Hope this helps.
Ed