3 Labradors - Genders? Ages?

I would say all the same gender. I don't know which gender would be better though because I've seen both male and female trios do well with each other, but don't get two males and one female or vise versa because they will fight for attention whether they are fixed or not( I have seen it before). The number one rule to having a good grouping is getting dogs a couple years apart. In that time the dog will have broken ut of the puppy phase, been completely potty trained, exposed to almost any situation, hopefully socialized with different kinds of people and dogs, and learned the pack order in your house. The best way to 'train' your dog to be dog friendly is to socialized it as soon as it is old enough for puppy kindergarten. Once the cl is over and the shots are UTD, take it out to parks(keep on leash), stores, and doggy daycare. If you want them to ignore dogs on walks I would get the dog enrolled into an obedience cl and ask the trainer. Good luck