Curd

Thank you all three! Such different but valuable insights. I'll let you know how it turns out.

It shouldn't matter as long as your total quantity remains the same - however some flavours will be sharper and more pronounced than others; I suggest you taste as you go, but remember to keep track of what you add - there's nothing worse than really liking the outcome and not remembering the ratios!

Here is what I do when I make a mixed citrus curd https://food52.com/recipes/15642-mixed-citrus-curd

I don't think it really matters -- it comes down to what flavors you'd like to emphasize at the end. Lemon or lime will drown out orange, so you might want to go heavier on the orange if you're using it and want it to shine through. Grapefruit is bitter, so bear that in mind in terms of thinking about proportions.