The Proto-Celtic word *tri
Word *tri
Meaning three, 3

Other words that might be of interest
*φelu ‘many’, *oino ‘one’, *dwei ‘two’, *dwā(w) ‘two’, *swexs ‘6, six’, *nawan ‘nine, 9’, *kʷetru ‘four’, *kʷenkʷe ‘five, 5’, *oljo ‘all’, *sextam ‘seven’