My current wiring for 3hp tablesaw is a double 20 breaker with 12/2 with ground. Currently 45 ft run. If I need to extent to 50-60 ft, can use a junction box and continue or re-wire back to panel with 12/3 with ground? This is a residential basement workshop.
Thanks,
5square

The use of a double 20 amp breaker implies that the saw is wired for 240 volts.
Three horse power is 2237.1 watts.
2237.1 watts divided by 240 volts is 9.32 amps.
Voltage drop calculator shows a 4 volt drop over 100 feet with 12 gauge wire at 9.32 amps.
That is a 1.7 per cent drop well with in voltage drop limits.

Use junction box to add 12/2 with ground cable to required distance.

Why exactly would you want to use 12/3.. unless you have an abundance of it?
Extending the circuit with the same wire is extended. The additional 5 to 15' you mention will not be an issue. Is the saw wired with a plug? If so you can leave the plug where it is and add another one at the end of your run.