Wait. We got our lab when ds was 9 years old. Felt like the right time.
Love him to bits but we seriously underestimated the commitment and running costs!
Roughly 2 x 1hr off lead walks a day, preferably in a few different locations to keep it interesting, come rain, snow, wind, winter evenings 7 days a week 52 weeks a year. I couldn't have done that with a 3 year old. Whatever both your social life/hobbies/work commitments are you will need to factor in someone walking the dog in the pissing rain while the other stays in with your 3 year old.
Even if you are lucky enough, because you cant plan it, to get a fabulous, placid, well trained dog you will never be able to leave the 3yr old alone with the puppy even to leave the room for a second, or with the unpredictable growing dog for a least a year or more.
Want to go to mums for Xmas dinner, need to work out what to do with dog as niece doesnt like dogs. Want to go to beach for day, need to take dog, ds wants to play footie whats happening with the dog, dc in hospital....dog?
The costs, if done right, are ridiculous, big ones are:
Insurance (6yr old lab) £54/month (will increase as he gets older)
Health care £11/month
Food £50/month
Walkers (2 days a week) £90/month
but the others add up too - weather proof clothes/boots for you/dh, treats, beds, vet bills that arent worth the £85 excess claiming on insurance etc etc