those 2x tyres are pretty close in size,
3.00 x12 is 75mm wide. 3" = 75mm
a 89/100 x12 is 90mm wide, with the sidewall being 100% the same height as the width.
there are conversion sheets in the web, use google, if you get a 80/100/12 it will fit. This tire converts to a 2.75 to 3.00 x 12. any bike store will guide in the right direction and tell you which tire to buy.