Maybe two of FiiO-E6? A few dollars over your budget (for two). I have not used them myself, but are recommended reviewed on http://lifehacker.com/five-best-portable-headphone-amplifiers-1722800379
Response to comment: well, they meet your requirements of being
- pocket-sized: "1.6 x 0.4 x 1.6 inches ; 0.6 ounces"
- battery powered: "1 Lithium ion batteries required. (included) [...] Built-in rechargeable battery provides up to 10 hours of normal use. Mini USB charger port can be used with any USB power source."
As for "standard headphone jack input", I'm guessing you want 3.5mm jack. The Amazon page fails to say the jack size, but in this video review you can see it's a 3.5mm jack.
Since you're using two devices, these following requirements
- at least 2 standard headphone outputs, preferably 3
- separate volume control for each output
are trivially met (2 outputs).
As for price, "$27.99 & FREE Shipping on orders over $35" at Amazon. So if you buy two right now... $56 shipped. (Since you already have one amp, you could just keep that and buy just one other single-output amp [this or another], saving some dough.)
People say (in those reviews) that reliability of FiiO products is mediocre though. The most geeky/in-depth review of it you can find is probably the one by NwAvGuy. His review can probably summarized by quoting this bit:
The E6 is not an especially clean or well behaved amplifier but in light of the very modest $30 price it’s likely acceptable for many applications for those on a budget.
Here comes the stuff you probably don't care about. The opamp (i.e. the chip that determines sound quality) inside this FiiO E6 is an AD8692 ($0.60 list price in 1K quantities). Most the parts cost goes into making this thing run a Li battery. The opamp is a surface mount (SMT) part, so you have to be pretty adept to replace it with something better in your garage, but some have pulled it off without professional SMT equipment.