Hi there and
to the forums!
I'm so glad you were able to rescue the little one and s/he seems to be in good health!
You're going to need to quarantine the new bird right away, for at least a month. Budgies are very good at hiding illnesses and since this little one has just come from the outdoors, it's especially important that you don't let him/her come into contact with your birds for another 40 days in order for all hidden illnesses to be ruled out as well as to let him/her settle into your home
Have you looked for its owners at all?
When the time comes to introduce them, you can let them play out of their cages together in order to judge the flock dynamics. If it seems like they all get along wonderfully and they start going into a cage together, etc. then they may do well in a large enough cage together. However, it's perfectly fine to house them separately, your male-female pair and the female in her own cage, but in the same room, and letting them play under supervision. It can be difficult keeping odd numbered budgies of different genders in one cage with regards to flock dynamics, but on the other hand, many members have birds who have worked things out and get on quite well
Ultimately, after a slow introduction, it's going to be up to them if they'd all like to live together.
I hope that helps!
Meanwhile, you've come to a great place to learn even more about the best of budgie care practices! The budgie articles and "stickies" (threads "stuck" to the top of each subforum for easy reference) have all the information you need to stay updated on everything. If you have any questions after reading through things, please be sure to ask as we'd love to help.
Cheers, and we hope to meet your budgies soon!