Group effect of piles in Plaxis 3D?

Group effects can mainly be categorized with respect to pile spacing. If piles are far enough apart then there is no evident soil-pile interaction. If piles are little closer together then there is minor soil-pile interaction. If piles are even more close enough together then there is significant soil-pile interaction.
An increasing in the number of piles in group, decreases the group efficiency. When considering piles that are close enough together to cause significant soil-pile interaction, the ultimate resistance for grouped piles may be lower than that of a single pile due to the overlap of the shear-failure planes from the movement of each pile.
Furthermore, consideration is to made for the interaction of piles in the direction of loading when the behaviour of leading and trailing piles will be much different. 
So its not just axial forces that you should be looking at, but also, shear stresses in soil, deformations in soils and bending moments in piles as well. This will give you an overall picture of how the pile group compares to individual piles. 

Group effects can mainly be categorized with respect to pile spacing. If piles are far enough apart then there is no evident soil-pile interaction. If piles are little closer together then there is minor soil-pile interaction. If piles are even more close enough together then there is significant soil-pile interaction.
An increasing in the number of piles in group, decreases the group efficiency. When considering piles that are close enough together to cause significant soil-pile interaction, the ultimate resistance for grouped piles may be lower than that of a single pile due to the overlap of the shear-failure planes from the movement of each pile.
Furthermore, consideration is to made for the interaction of piles in the direction of loading when the behaviour of leading and trailing piles will be much different. 
So its not just axial forces that you should be looking at, but also, shear stresses in soil, deformations in soils and bending moments in piles as well. This will give you an overall picture of how the pile group compares to individual piles. 

how to check the shear stress in plaxis?

In PLAXIS output, in the Stresses tab, you can expand the principal effective stresses and there you can select Mobilized Shear Strength and Relative Shear Stress. The mobilized shear strength is the maximum value of shear stress (i.e. radius of the Mohr stress circle or half the maximum principal stress difference). The relative shear stress gives an indication of the proximity of the stress point to the failure envelop.