Threesome?

So my husband and I haven’t been having the best sex lately and my husband brought up having a threesome with his acquaintance (I’ll call him D). At first we talked about it as a joke but now when me & my husband have sex we role play like we’re having a threesome with D. D is all for it but I’m worried my husband and I are having a psychotic break 😅 someone validate my feelings please!!