class A1 // Может быть A2 A3 etc
{
    public function b(): BelongsToMany
    {
        return $this->belongsToMany(B::class, 'entity_bs', 'entity_id', 'b_id')
            ->where('entity_type', EntityType::A)
            ->withPivot('*')
            ->withTimestamps();
    }
}

class B
{
    public function roles(): BelongsToMany
    {
        return $this->belongsToMany(Role::class, 'b_roles', 'b_id', 'role_id')
            ->withPivot('*')
            ->withTimestamps();
    }
}

class Role
{
  // code
}

pivot table (entity_bs)
id entity_type(A1, A2 etc) entity_id b_id
pivot table (b_roles)
id entity_type(A1, A2 etc) entity_id b_id role_id