Looking at my MEL, there are 3 different detection loops
Look again. Unless you fly an unusual type, there are probably two loops that have 3 sensing elements (nacelle, turbine and core). So if one loop is MEL and the other goes during the flight, you have no fire sensing capability left. Apologies if your aircraft truly has three independent loop/detectors!
Wouldn't the second loop fault (with the first already inop) in fact give you a fire warning?
On the A320 series the fire warning is inhibited if both loops fail within more than 5 seconds of each other.
P