three cannons
A specific counting expression using the counter 門 for cannons or large guns.
Three cannons were installed on the castle walls.
大砲 means 'cannon' or 'artillery' in general; 大砲三門 specifically counts three cannons using the counter 門.
門 is a counter for cannons and large guns; it is not used for small firearms.
Compound of 大砲 (cannon) + 三 (three) + 門 (counter for cannons). The counter 門 is historically associated with large objects like cannons and gates.