We found going from one to two very easy, slightly harder going from two to three
Obviously you need to consider the financial aspect - there'll be the aspects of insurance, kennel costs when you go on holiday, and of course food. It also depends on whether or not you regularly use a dog walker (so extra costs) and of course the size of your existing car. Having said that we used to have a lovely friend who squeezed two greyhounds into a Ford Ka - never quite sure how that happened!
In terms of walking and social side, it was a breeze - we did the same amount of walking (just with more poo bags!) and they loved spending time with each other. I think what made it easy is that they were both adult dogs with similar requirements (both greyhounds) so easy to fit in together. I think it might be more of a challenge when dogs are different breeds/big age gap/one is a pup etc.
Also it depends on your set up in terms of looking after when you have occasional days out - it is definitely harder to find people willing to dog sit three (or even two) dogs as opposed to one, and your options are more restricted when looking for things like holiday cottages.
Due to bereavement, we've recently gone down from three to two, and it feels ridicolously easy, and we are already considering options for a third dog e.g. foster etc..
Fostering is actually a nice way of dipping your toe in the water. You can try it, you know you are helping a dog into a forever home, and you are not saddled with a permanent extra dog if it is not for you. Also the rescue will cover food and vets bills - might be worth thinking about? 