I have difficulties in taking power chords in Drop D tuning. Basically, in standard tuning I would mute other strings with my index finger, but here I have difficulty in barring three strings and mute other three. Can you suggest something?
With the electric guitar, I make the power chords using just the index finger to press the 6th, 5th and 4th strings. With the rest of your index finger you can mute the top three strings, and you still have the other fingers to add more notes if you ever need them.
It might be a little more difficult doing that on an acoustic guitar because of the string tension, but I still think it's the easiest way.
You might have to change a bit the way you're strumming the strings. As you're only playing on the bottom 3 strings, you should focus on your strumming hand in order to reduce your movement, maybe you go too far. This is not a "full chord" strumming, you should stop your movement sooner in order to avoid touching the 3 string you don't want to play.
Moreover you just have to train in order to release pressure on the other 3 strings in order to mute them with the same finger (the same exercise you need to do to play a proper A chord with one finger).
Combine this "light" muting and a reduced strumming movement and you should be able to play nice Drop D powerchords.
it just takes practice, only use your first finger, lay it across all six strings and practice pushing down the top three strings and resting the rest of your finger against the higher strings so they're muted.
Your finger just needs time to learn the position - it's called muscle memory. You can do it with multiple fingers but it'll be slower.
Also focus on just strumming the top three strings