For my Scion tC, I have a "3" gear. I know D is drive and R is reverse and N is neutral.. what is "3" used for

3 is most likely to lock out overdrive. The ones that say low and L2 are actually 1st and second gear lockouts that lock the gears until you shift. One guy said to pull hills, but you would be suprised how good an automatic takes off in ice and snow in 2nd gear.