Man pages matching name fltk in section 3:

fltk.3.en	debian 10 / libfltk1.1-dev
fltk.3.en	debian 10 / libfltk1.3-dev
fltk.3.en	debian 11 / libfltk1.1-dev
fltk.3.en	debian 11 / libfltk1.3-dev
fltk.3.en	debian 12 / libfltk1.1-dev
fltk.3.en	debian 12 / libfltk1.3-dev