I must be one of the greedy ones.
Increase the price, but do it sensibly. The conversation with me would go something like:
"You've asked me to clean every three months; I'm sure you'll agree that there's a bit more to do if I clean them with that gap. To some extent, monthly is keeping them clean, while at three months, I'll be cleaning off dirt. In light of that, I'll have to add an extra £x to cover the extra time I'll be here".
That all sounds very formal, but I'd do it as a very light informal chat. No-one has ever disagreed with that line. Ever. They will immediately understand that three monthly cleans must be more work than four weekly, and they will expect to pay more. If it looked like I might lose the business I might move on the price, but it's never, ever been a problem.
I understand that saying that on here will bring out the "You're a greedy git" brigade, but I'm in business, not charity. And your customers aren't fools. They expect to pay more for more work. Don't disappoint them.
Vin