I am planning to travel from Tbilisi to Baku & Yerevan in mid-July. I visited the https://tickets.railway.ge but I cannot find the two sites in the destination list.
The train tickets from Georgia to Armenia and Azerbaijan can only be bought in person with your passport. They are not available online.
I will be in Georgia 3 days before the first trip
Is there going to be any availability?
I don't know but probably yes. 3 days in advance should be enough.
Yerevan is easier, even if no train tickets available, you can take a minibus/marshutka, 4-5 hours drive. Baku is much farther.