This is a long one, but I really hope you'll take the time to read it!
How much have you considered the overpopulation issue? I'm surprised no one else has mentioned it. Especially in these times of food shortages, climate change, wars, environmental disasters, etc, I think we should all give very serious thought to how many more people we want to bring to this already very taxed planet. Please check out www.vhemt.org (voluntary human extinction movement), they have some excellent (and humorous too) information, including a list of reasons why people want to have babies and other ways you can satisfy those needs without bringing more people on board this fragile planet.
Your boys already have each other, and I'd encourage you to continue to develop more social connections, so they can have other friends as well. If you want to have more children around you, there are many ways to satisfy that desire. There are so many children already born who desperately need love. I imagine there are children near you whose parents don't have as much time or energy as you do, who would love to join your family for camping trips and other fun times. There are children looking for forever homes through adoption, or you could take in foster children, or lead nature walks, help out at schools etc.
We humans have two arms, just enough to hold (or hold onto/hold hands with) two children! You can devote much more time and attention to each child, the fewer you have. Obviously if you have more, your heart will expand so that you can love them all, to the best of your ability, but there is a limit to how much energy you can give, when you have to spread it out more thinly.
To share my experience - I loved having babies so much, I wanted to keep having them as long as I possibly could (I originally wanted to have at least four, so they could be a string quartet!). After my second (who was born 11 years after my first, so that each of them had the luxury of being an only child, receiving my full attention), I tried for five years to get pregnant again, and finally did but miscarried. It was a long grieving process for me to eventually accept that I would have no more babies. For me, in a sense having babies was like an addiction to a drug, I so loved the feeling of being needed, having someone to cuddle, along with the oxytocin and prolactin hormonal rush from pregnancy and breastfeeding. Now, however, especially as I know more about overpopulation, I am grateful that my body stopped at two, and I realized how my desire for more babies was an attempt to meet my own unmet needs for love. I have been through some good therapy and have found other ways to give and receive love and to feel needed. I do now have three wonderful grandchildren, which is a blessing.
You didn't mention it, but I wonder how much of your desire for a third child is wanting to have a girl? If so, of course you realize there is no guarantee that would happen, the chance would still be 50/50. Also, the fact that you have a career (which I assume you enjoy) would also seem like a good reason not to have another child. I'd encourage you to think seriously about how much time you could spend with each child after a day of work, and how that would affect them.
I realize I am expressing my opinions pretty strongly here. I don't pass judgment on people who have more than two children (it could have been me, if things had worked out the way I wanted!). Also, on the other hand, from a spiritual perspective, I believe that it is possible that sometimes another soul may really want to come to earth at a particular time through a particular family, even though the parents had thought that they were done having children. I have heard that sometimes the soul will come to the mother through a dream or vision or intuition, asking to incarnate. And sometimes they come even though the parents are using birth control, and then it seems clear that they were "meant to be." Another possible viewpoint in favor of having more children could be that the world at this point really needs some good caring leaders to help us out of the mess we have gotten ourselves into. But - I think that in general, rather than bringing more people here to accomplish that, it makes much more sense to give all the love we can to those who are already here, so that they can grow and mature in love and become the strong caring leaders that we need.
I encourage you to really think this through, pay attention to all your feelings, discover your underlying needs, and consider all of these factors before making a decision. And if you do decide that you want to have a third child, I still think it might be better to wait longer, so that each child can have more attention. (I think it could be especially hard on your younger boy to become the "middle child," who can end up feeling lost between the other two). Best wishes to you in your decision.