Real Madrid performed poorly in La Liga last season but played well in the Champions League, losing to the reigning champions in the semi-finals. So it may happen that this season they may do well in La Liga but poorly in the Champions League. We have to wait for the group stage and see how they perform.
Before the group stage begins, it is indeed difficult to determine the strength of a team this season in the Champions League, even though the domestic league has already started. And even though the domestic league has started, it doesn't mean that what you see now is the real strength of a team like Real Madrid, because it could be that in the middle of this season, Real Madrid has another strategy that they didn't apply last season, so they might be stronger. Because it's still not certain that Real Madrid can be stronger in La Liga, but it could be worse in the Champions League, even though for now Real Madrid is at the top of the La Liga standings after wading through three matches.