Can someone please advise on conditions/diseases that have multiple skin involvement (psoriasis, dermatitis, eczema all at the same time)?

Unclear question.: Do you mean whether one can have (atopic) eczema, dermatitis, and psoriasis at the same time? Since eczema is used loosely, it may mean a skin rash including allergic contact or irritant dermatitis, then the answer is that it can exist with psoriasis or atopic dermatitis (the latter actually increases the risk in developing contact dermatitis). Psoriasis does not co-exist with atopic eczema.