Is there any good oral medicine for a 3 year old constipated toddler?

Try natural methods : Try natural methods first such as a glass of water, juice, or a naturally hydrated fruit such as pears, prunes, etc. If this fails you should consult you pediatrician before giving anything over the counter.

Chronic constipation: Requires medical management for at least 6 mos. For both health & behavioral reasons. His pediatrician can determine if he needs a "clean-out" before giving MiraLAX (polyethylene glycol) to achieve a soft stool at least every other day, at which time you can put him on the potty for 15 minutes, 15 minutes after a meal, with his feet on a stool so his knees are slightly flexed. Let him blow bubbles, color or read.