Moving in with friends

So my best friend of 13 years and I have been planning to get an apartment together for the past few years and this summer we have it planned to move in together. We’ve seen some places that we have liked and picked out decor and everything. My other best friend all of a sudden showed interest that she wants to move with us too now. She asked me so randomly I didn’t know how to react so I just said “sure”. Both me and my friend I’m moving in with don’t want to live with her and wanted it to be just us two like we’ve planned since we were little, but I don’t know how to tell my other friend that because she is super sensitive and takes everything to heart.. part of me would rather just suck it up and get a 3 bedroom apartment instead so she can live with us instead of telling her no!