@Nina Thanks for the question, I'll try to answer in the best way possible so you can see both the pro's and con's for this set-up :)
Setting up your property with single occupancy as a standard has a few advantages, the first one being the age based pricing that is available. Secondly, it allows guest to book certain rooms with fewer guests and benefit a little from a slightly lower price. However, it isn't the only way to make age based pricing available. When there is a separate age category for adults, you can set up the adjustment not only for children based on double occupancy, but also for the adults🎉 When guests then select the number of adults they'll have a different adjustment then children.
The reason this might be preferred is because with single occupancy pricing your revenue on busy days might be impacted. Let me explain a little further, in this example we'll look at extra bed adjustments instead of empty bed adjustments for single occupancy set-up.
Let's say the standard room night during low season costs €100 and your extra bed adjustment is €20, when booked by 2 guests on a low season day the total price will be €120. However, during a high season night your standard room price might be €150, and the extra guest would still be those €20, instead of it being higher according to the season.
When you are opting for double occupancy set-up these prices can be adjusted to the season in the pricing section of the rates, whereas empty or extra bed adjustments are a fixed amount regardless of the season. The ideal situation would be a way to set up a relative adjustment to extra or negative occupancy, this way the adjustment would increase or decrease depending on the season.
To answer your question about the possibility to book for 3 guests when using single occupancy, this would still work. But as you can see from the example, the amount for extra beds is fixed. Which means in my low season example you would receive a total of €140 for the room (€100 + €20 + €20) and during high season €190 (€150 + €20 + €20).
Meaning you have to consider if the seasonality plays a big role in your property and if it would be worth it to switch to this set-up.
I hope this helps you make a decision!