I think all 3 of the bags you mention have different vibes so we could easily make an argument for owning each of them....

But seriously, do you enjoy using your margaux? It offers a cross body option that the garden party will not if that matters to you. If your margaux is rarely used, definitely sell and buy something you with love and use. Think about it for a bit since margaux bags are hard to source lately and you want to be certain before parting with it. The garden party will always be around if you decide to add it. Good luck deciding!