Is three a crowd or is three company?

I have 2 beagles (5 year old female and 2 year old male). We lost our 15 year old pomchi female but have always had 3 dogs. We decided to foster and then adopted a 2 year old terrier mix female. She loves playing with the male beagle but her and the female don’t really interact. If anything, the beagle will bark at her when she is playing with the male. To be fair, she’s never really played much maybe 15 minutes after dinner or so. I have been giving her extra attention because I feel bad. Am I projecting emotions onto her? Do dogs feel left out? Is this going to get better? Not sure what to do.