It only spreads if you itch it and spread the oil that caused the reaction in the first place. You can WASH it, but do NOT scrub it - you want to avoid opening the bumps, because then they can get infected.

Do NOT itch. Do wash whatever clothes you were wearing when you were weeding, and throw out the gloves if you were wearing any and they aren't washable.

The best remedy I know is old-fashioned calomine lotion (they make it clear now, you don't have to use the hideous pink stuff), and NO SCRATCHING!

This reaction is mild, because it is your first reaction/exposure. The next time, your reaction will likely be worse, so watch out for those "leaves of three" and "let them be!"