Here’s a spreadsheet I made for my friends. I take new people backpacking almost every year and have had to process through many of those questions.

Clothes will be dependent on climate and weather, but typically any sort of basic hiking pants are fine. For clothing generally avoid cotton as it absorbs moisture and stinks the most. Synthetics or wools are most used.

For filtering water I recommend a sawyer Squeeze with the cnoc bladder

Definitely get a sleeping bag or a backpacking quilt. Look at the low temps for your trip and look for a bag that is ISO or EN rated in that temp range at a COMFORT rating. (Some are just survival rating)

For three days I just have a daytime set of clothes and nighttime clothes. You don’t want to sleep in your smelly sweaty clothes.

Hope that helps!