I'm not sure about the 10-12 part, but you could just specify 10,11,12 instead. Also, the day of week value has Sunday at 0, so 2 would be Tuesday.
*/5 10,11,12 * * 2
If you want to test it, just add a dummy entry like so:
*/5 10,11,12 * * 2 echo $(date) >> /home/user/crontest.out
Replacing the 2 for the day of week with the current day. Then just check crontest.out after the day is over to see which times were run.