If they are all 4 ohm SVC subs, then you would be wiring them down to 1.33 ohms at the amp. If your amp was stable at 1 ohm, then definitely. 1,2,4 ohms are just common and easy to compare, but not required to be a hole number. It might work, and it might not. I would expect an ALPINE amp to be well made, get really close to rated power, but MORE THAN MOST AMPS, I WOULDN'T expect it to do what it's not rated for. I would also expect that you aren't going to get much more than rated power out of it, to try to turn it up for 3 subs.
If those subs are 500w RMS, I would rather have (2) 10s getting the wattage they are supposed to get then 3 subs getting 2/3rds.
IMO your two options are:
YOU HAVE TO USE THAT AMP: Take out the middle sub, and see what you can do with that space. Maybe you can turn it into a port, a passive radiator, or maybe even an amp rack. You could use your amp, and that's still going to hit pretty well for what it is.
OR IMO THE BETTER OPTION, IS TO JUST GET AN AMP THAT WILL PLAY ALL 3. You would want 1500w RMS @ 1.33 ohms by 1ch. They will never rate that, but when you do the math 1.33 is right in the middle of 1 and 2 ohms. If it can do 1700 @ 1 ohm, and 1300 @ 2 ohm, then that would be right there at 1500w @ 1.33. Anything that can do 1500w @ 2 ohm and is stable at 1 ohm would work, (and they aren't hard to find, or cost that much).
I know you're other thread you were asking about the electrical to support it. That might be the reason you don't get the third sub, (or just not yet). That 500w sub is adding about 40-45 amp draw on your system. Maybe you find an alternator that's about 220 for a great deal. Maybe a 260, 280, or 300 amp alt is the next option, a few hundred bucks more and with the cost of a new amp is not in the budget right now. An amp you can buy and you can sell it or maybe find a different use for it. (unless you've got 2 or 3 vehicles with the same engine, and same alt), Alternators, SUK buying one just one size not big enough, and then buying another one. The first one you buy should be big enough for what you MIGHT want to do.
An alternator has two numbers that matter when buying one:
amp output at or near idle, &
rpm needed for max output. You need to know what it will do when you stop at a light, and what will it do when cruising. Know what RPM you cruise at, and get those two numbers before you buy. If they can't or won't give em to you, then move on. Any great alt will do most at idle, and full output around 2K rpm. The
ebay specials you see for $100 bucks or so will do less than your stock at idle, and won't do full until over 3K rpm. IMO if budget is a factor, then get the good alt first, (that is big enough for what you MIGHT do). Do your 2 subs with that amp, and enjoy it. Keep an eye out, maybe even try to sell your amp while you're enjoying it, and use that cash for a new amp. Then get the third sub going.