Hi and welcome to nameberry.
Your daughter will have two lovely brothers named Travis and Tristan and therefore you were looking to keep the Tr theme for your daughter. I think that with your sons you have chosen fairly traditional names and they work well together. If you choose Trinity I think that the tryndiness of the name spoils the trio a little.
I feel that Felicity Jade would make a lovely fit eg Travis, Tristan and Felicity. Don't you feel that it has a lovely feel to it?
If you were to say keep a T theme then there are some great names for you to select eg
Tessa I think she is instantly likeable
Tamara is a gentle name, fully of feminity and sweetness
Talia or Tahlia is an especially good modern choice, it is unusual but recognised and is a lovely feminine name which I prefer to see after having two brothers.
Tamsin (the feminine form of Thomas) is unusual and semi unisex, as is Tegan but neither of these would be my choice.
Thea is a lovely choise too and very classic
I am sorry to hear that you have been getting some grief on nameboards but hear on nameberry we always try to give you good advice. I hope that you can understand where I am coming from and please let me know if you like Felicity Jade.
Hi, I don't mind Trinity but agree that Tristan and Trinity are too close. What are their nns? Might make a difference.
Tristan, Travis and Tracey? (or Tracie)
What about using the T but not the TR?
Tristan, Travis and Tara (Tarah)
Tristan, Travis and Taren (Taran)
Tristan, Travis and Terentia
Tristan, Travis and Teresa (Tess for nn)