SGA3 and [the œuvre of Bruhat–Tits](https://mathscinet.ams.org/mathscinet/publications-search?query=au%3ABruhat+au%3ATits) are both quite daunting.  The remedy has been, with encouragement from wiser colleagues (thanks, Brian Conrad, Sean Cotner, and Stephen DeBacker, as well as anyone else I've forgotten!), just to sit down and read them, and find that they are by and large beautifully written.  For SGA3, this is helped immensely by the lovely [réedition by Gille and Polo](https://webusers.imj-prg.fr/~patrick.polo/SGA3/).  For Bruhat and Tits, J.-K. Yu's introduction [Bruhat–Tits theory and buildings](https://doi.org/10.1090/fim/026/02) was a wonderful roadmap.