If I get paid on the 1st, and the 15th of each month in 2007, what months would I receive a 3 week payroll check?

Never. If you are getting paid on the 1st and 15 you are getting 24 pay periods in a year. Divide your total yearly income by 24 to arrive at what each pay period pay should be. You may be thinking your paid every two weeks, in which case you would get 3 checks in one month. That is not how you said you were getting paid. Twice a month means you get the same pay in Feb (with 28 days) as in March with 31 days.

wags, I guess on months where there are to 15ths of a month.

Yes you are confusing this with getting paid every two weeks, which it is not, there will be longer periods ( more days ) from the 15th to the 1st.
But you will as noted never get 3 checks in a month.

I hated this, I used to be paid the 1st and 15th and I got 1000 twice a month every month (I made 24000 at the time) now I'm getting paid every 2 weeks and every now and then you get 3 paychecks in the same month and it is awesome!

Ummm yeah, but as said above, never, they divide your pay into equal payments