I ran into this dilemma once with a table saw.
Place I used to work had a big Powermatic table saw.
It had 3 belts on it as a matched set.
If one of the belts broke it was ok, we could still use the saw.
But when the second belt broke we had to replace all 3.
But we saved the 3 belt for later use.
I was told that the belts become stretched over time.
And they all 3 stretch approximately equally so they stay matched.
But for putting new belts on, we had to put 3 new belts on at the same time.
Except:
we saved the old unbroken belts, and when we got 3 of them,
we could put those 3 on the saw and get a little more life out of them.
It just took a few minutes to change belts on the saw.
And this was more cost effective than just trashing them.
You mite want to do the same for your finish mower.
Pooh Bear