Thanks for all the great input guys, it seems the threesome thing is quite contentious! It is something that we've discussed before but I get that it can be quite different to a poly situation, just like 1 night stands are different to long term relationships. I'm still not sure whether I would want to engage with other people but doesn't seem as alien as it once did, can I ask what people's opinions on primary/secondary situations are? Is it possible to work like that or for a healthy situation is equality necessary?
It can work fine if everyone is on the same page. My gf and I each have had "secondaries" where there was or is love between partners, but not as much life entanglement.
You might read this
https://www.morethantwo.com/polyforsecondaries.html