Do not create new topic for every post!
GxEPD2_3C<GxEPD2_750c, GxEPD2_750c::HEIGHT> display(GxEPD2_750c(/CS=5/ SS, /DC=/ 22, /RST=/ 21, /BUSY=/ 4));
Change to use with your 2.9" 3-color:
GxEPD2_3C<GxEPD2_290c, GxEPD2_290c::HEIGHT> display(GxEPD2_290c(/*CS=5*/ SS, /*DC=*/ 22, /*RST=*/ 21, /*BUSY=*/ 4));
This is my last post for you, unless you acknowledge my post.