I agree with the advice you received. I have kept city chickens for many years and I find the hardest part is finding the right balance of # of chickens to meet my egg demand, because to achieve this balance requires culling or retiring older hens and introducing new ones.
What makes it challenging for me is that I don't buy store eggs so rely on my own eggs for vegetarian protein, and I don't cull older birds - I let them retire. This means I have to introduce new birds every few years, and although I love getting new chicks/hens, I dread the integration process. You can read of the many challenges with integrating chickens on BYC. Although most are successful with time, it is almost always challenging and sometimes heart breaking to see how cruel chickens can be. The amount of fighting depends a lot on their temperament and how much space you have - and for city folk we typically don't have much space and fighting can be extreme.
So, I would get at least one more now of the same age. I think 3 is a good number if 1-3 eggs/ day for a few years sounds right to you. 3 chickens are not much more work than 2. It is a very real possibility that you will lose one and then you'd be in a bind. If you don't want to replace older birds, odds are in 3-4 years you will have 2-3 older hens, and you could add 2 more to compensate for egg production. As time goes on you might get an excess of older birds / too few eggs and you'll have to make decisions on how to deal with that. I'm hoping that by the time that happens to me I'll be in the country and have more space!
The last time I added to my flock, I had 3 older birds and should have gotten 2 to keep up the egg production. But of course I got 4 chicks at the feedstore, because I figured 1 might die and 1 might be a rooster (this was a chicken-aholic rationalization!). Ugh - now I have 7 chickens. Although i have the space it is a few too many for me. Next time I add, I plan to get few ~6 month old hens so that I know they aren't roosters and so I know they are past the infant mortality phase. But since yours are young it will be MUCH easier to add another young one now.
Many chicken experts advise against introducing new hens every few years as it causes a lot of stress, fighting, etc. and raises the risk of introducing disease to your flock. This is sound advice from the 'chickens as livestock' perspective. But if you don't introduce new hens, this then you need to cull or have a bunch of separate coops and that is not an option for me.
Good luck with your new chickies.