In the UK there is infrastructure sharing, (EE/Three and Vodafone/o2) so the max would actually be 2. You also wouldn’t have 3 sectors in a setup against the wall, but even if you did it is possible to have slim antenna where 2 or 3 panels fits into a single enclosure.
Certain areas even have all 4 networks sharing a single mast. For example:
This mast is running 2G, 3G and 4G for EE/3/Voda and O2.
There’s an even slimmer version.
This is dual network o2 and Vodafone, capable of 2G 900, 3G 900/2100 and 4G 800/1800.