I'm using the mini version of that case. I have two 140mm fans in the front and one 120mm in the rear. I would put two fans in the front but mount them as low as possible to the PSU shroud and just have one in the rear, anymore than that is just adding extra noise for no real reason.
If you really do need to add an extra fan after that, just add one to the top of the case, But I doubt that you will seen as the Meshify has plenty of ventilation anyway.