Sentinel-3 LST mosaicking

You can use the collocation tool. Which tool is best depends on what you want to do afterwards.

You can also convert all the tie-point grids into bands using BandMaths in preprocessing step.
This can be done via the command line for a batch of products with gpt or in the GUI but then it is tedious work if you have multiple products. If you do it in the GUI, ensure to uncheck the Virtual option.
I’ve mentioned this approach here: Export of conc_tsm after C2RCC processing - #2 by marpet