My seat is in the third row.
eel MEE-oh POS-toh eh een TEHR-tsah FEE-lah.
When looking for your seat inside the theatre and explaining to an usher or fellow theatre-goer where you are supposed to sit.
Il mio posto is my seat or my place. In terza fila means in the third row. Fila is row. The ordinal numbers are used: prima, seconda, terza, quarta (first, second, third, fourth). Posto can also mean place in general.
Qual è il mio numero di posto?
What is my seat number?
When checking the ticket for the exact seat.
Scusi, occupo questo posto.
Excuse me, I believe I have this seat.
When someone is in your assigned seat.
Posso sedermi qui?
Can I sit here?
When asking permission for an unnumbered seat.
Italian theatres typically number seats from left to right facing the stage, with odd numbers on the left and even numbers on the right in some houses, or consecutively in others. Ushers (maschere) will always help you find your seat.