The medbay is a production room that unlocks at a vault population of 14 dwellers. Dwellers will produce stimpaks in them. Medbays also extend the vault's storage capacities for Stimpaks. Medbays depend on the Intelligence trait of the dwellers working in it.
The output of 3 upgraded double rooms is higher than the output of 2 upgraded triple rooms. The storage capacity and the used space in the vault is the same. That means 3 double rooms are more efficient than 2 triple rooms.