Apple's implementation of TB3 on the MacBook pro (my only experience) can only do one 4k monitor at a time.
There are many TB3 docks available like https://mymantiz.com/products/md-01-zeus that offer dual 4k @60 Hz and as far as I know, they work just fine on Macbook Pro.
Are you sure you're not confusing it with USB-C, which can only drive one 4k@60Hz monitor per connector? USB-C connectors are compatible with TB3.