Hello and welcome to BB!
I have a bonded trio and they are AMAZING to watch together. I also have two boys and a girl! The boys were inseparable from the moment they met while my girl had to get to know the boys on HER terms... and only then would she begin the bonding process with them! LOL. If you decide to get a female, I would suggest an already spayed female. My girl was unspayed when we introduced her to our boys and BOY was she just a little brat. She thumped at them a million times and would bunny-butt them every time they tried to come up to her! Then finally she became curious and started to follow them around sniffing. Hahaha.
I would at least wait another 2 weeks if you plan on bringing another bun in to ensure ALL hormones are gone between all so there's no hormone aggression issues. A MUST is you have to bond each pair separately before putting all 3 together. I would suggest getting a third bun ONLY if you have the time, energy, and patience into giving the bonding process the time it needs.
The cost of the third rabbit really depends on your area. You, of course, have to add in the cost of any extra vet visits for the new little girl but as for feed, we just began buying the bigger bag. Since our 3 has turned 7 months and now get limited pellets the pellet intake isn't very expensive. (A TON less than when they were babies.) There is of course the added cost of hay -- but we pretty much completely eliminated the extra hay by buying much bigger bags (Oxbow) to save money. We now spent about $2 for a pound of hay rather than the $5 we were originally spending so it's actually cut down on our hay cost if you can believe it.
If you give them veggies -- then that cost will obviously increase. My buns all get a cup of veggies a piece a day and we spend about $7 a week on all 3. And last but not least, litter! Our litter increased because we began using CareFresh but we had made the mistake of putting it down as bedding also so we were flying through it. Now that we ONLY use it in the litter box it's a lot cheaper. How is your bunny set-up now? We had 3 cages but ended up building (a much cheaper AND bigger) bunny condo when they were all bonded. There are a ton of cheap ideas for building bunny houses and extra bunny toys on the forums also! You will be adding on the SAME money that you do for each bun -- but we elimiinated that cost by searching around for cheaper options and now end up saving more money than we originally were with two! It depends on how you budget and look it at the cost of each of your items and how you can buy in bulk now to make up for the extra cost.
Hope this helps -- if I missed anything or you have any other questions feel free to ask! I love answering questions about the bonding of the buns and this website has really given me lots of ideas and thoughts on saving money for the buns!
Also, pics are a must when you post on here! Would love to see pics of your little boys! How old are they? And what breed?! Squeeallllll. New bunnies to gawk over!