I took it upon myself to think two straight girls making out and about to have sex was an automatic invitation to a threesome
What made you think that they were inviting you?
Because they said the two of them wanted to make out and felt like doing it together with me at the same time
I think that you should ask them if you want to join. You should not just invite yourself if you think that they want you. Communication is key!
At least they let me join in. I love watching girls make out.
Things might have ended well for you this time but its better to get consent before just joining in. Next time things may turn out badly for you. You wouldn't want to get in trouble or make things awkward.