Hi, Mrs S.Shrimply, your question isn't this easy to answer and it raises questions, that need to be answered first... Sumps are far from standardized products and all are DIY projects or Custom builds that fit the situation. And all are built according to the builder's previous experiences in how a sump should be equipped and divided. In this, there are many different opinions and a rather big choice in different materials to use and routes to go.
Building one from scratch for the very first time without any experience is particularly challenging and a lot of trial and error, 9 out of 10 times it requires some changes and tuning afterwards to address unforeseen issues in how it functions. The first sump is mainly built without experiences from imagination, it all can seem very logical. But water can run in mysterious ways if subdued by natural forces playing up you didn't think about.
1 tank 1 sump can be pretty straightforward and a sump that is 25% of the volume of the tank it needs to filter could be more than big enough.
Water needs to go to and come from the sump it will have 1 overflow maybe 2, the sump will house 1 pump and 1 or 2 heaters and the sump will be situated directly below the tank. And search the forum and find how many questions this raises by people to make them properly for the first time.
Then 3 x 60-litre tanks on 1 sump complicate it a bit and is a totally different situation. It's jumping in the deep before having your first swimming lesson if you have no sump building experience. In my imagination, I could go too many routes with this and still be pretty far from the idea that is in your head. I wouldn't know where to begin answering your question without knowing and personally seeing the situation it all needs to be in.
What's the idea? 1 sump but 3 pumps? Then you need enough room for this. 1 pump feeding 3 tanks? Can be done theoretically, practically I have not done this yet. Thus there we are at trial and error again. 3 or more overflows to 1 sump also can be done, but where and how are the tanks situated? What turnover is in the tanks you are aiming for, overflows, pump(s) and sump should be calculated and sized accordingly?
When it comes to sizing the sump itself, in general, if you are not into DIY tank building or ordering one custom made you are kinda stuck with the standard sizes and volumes available in the LFS. That goes from 40x25x25cm (25L) to 60x30x30cm(60L), 80x40x40cm(128L), 100x50x50cm(250L) etc. etc.
If 25L doesn't do it, then all you have is the next best option 60L and make the best out of it and so on.
Anyway, I hope you get the idea now, why your question that is set up in 1 simple sentence is rather difficult or rather to say not at all to answer in a nutshell.