YOU are correct.......
I would tomorrow send a certified letter to the landlord stating the landlord has violated the lease terms, by changing it in the middle of the lease.
And that by releasing the third tenant out of thier LEGAL obligation to pay thier 1/3 share of the rent, the landlord has agreed to allow the other 2 tenants pay their 1/3 share as FULL rent for each and every month they are without a 3 tenant on the lease.
I think you have good legal grounds to demand the landlord take FULL responsibility for the 3 rd tenant, becuase it would be ILLEGAL for the landlord to demand you pay for their share... since they are not on the lease and the original lease was for 3 people not 2.
YOU had no say in this matter, since you know the 3rd tenat would be legally liable for any rent and damages until you rerent the room.
But the landlord took away your rights, and should pay for that!
The Landlord will be very pisssed at you..... but stand your ground.