It's a hardcoded value which you can freely choose in other implementations for each channel separately. Three is the most common value as far as I remember.
Shouldn't this be just a default option? Not a hardcoded value. Same thing for the minimum amount of 2 mBTC.Did you make sure to spend some coins from Alice's channel? As I wrote in my previous post, you won't be able to receive any coins unless you make some room for incoming funds first. You can use the testnet version of yalls.org for that. Also, you can double-click on a channel and you should see how much exactly you can receive through that channel.
The channel's capacity is 3 mBTC and it says that I can send 2.96416 mBTC. I sent 0.0015 mBTC on a testnet service (?) and it seems it worked:
Once I did, I closed the channel afterwards. Should I? This is the transaction that was broadcasted when I did: 46b6b2cc6570263a3a21a0ce20612d8952f5bd9a6ffa1914a14860c328270500
It's a multi-signature address sending 0.00299665 TBTC to one of my change addresses. Shouldn't there be an extra output with 0.0015 mBTC?