DP and I have been together for 2 years, very happy and often talk about the future. For context, he is 27 and I am 24.
This year, DP bought his first flat (2 bedrooms) and invited his best friend to move in with him.
DP recently brought up the topic of us moving in together, not in the immediate future but sometime in the next couple of years. He mentioned that he had briefly mentioned it to his best friend, and whether he would mind living with a couple when DP and I decided it was the right time to move in together. It wasn't a serious conversation between them, but his friend said he wouldn't mind living with a couple.
DP has asked for my view in the situation. I really like his friend, we get on very well. However, if I were to move in with DP, I would not want his friend to live with us long term. DP asked how long I meant by not long term- I said, maximum 6 months (thinking that was being generous!). DP said he was thinking more like a couple of years....of the 3 of us living together.
I would worry that living together with DP's friend could cause resentment from any of us, and ruin any of our friendships. I would worry that I couldn't be affectionate with DP in case the friend feels uncomfortable (at the moment the friend will eat dinner in his bedroom if DP are eating at the table in the kitchen). I would worry I would start to see DP as a flatmate rather than my partner.
I would want it to be our space- neither of us have lived with a partner before and I know it will be a huge change for our relationship and a massive adjustment, without throwing another person in the mix.
The conversation ended well between DP and I - he was very understanding and saw my perspective, and said it had given him good things to think about.
Sorry that this has been long!