That is the standard size “flight cage”. Yes, it is fine for 3 budgies. But... keep in mind, it’s actually a better home for two buddies who get along well. Many times, when you have 3, two of them will bond and leave the third one out to get pushed around and picked on.
How about getting that cage for the two you have, and use your existing cage for the new one? (Then AFTER quarantine the cages can go in the same room).