Say, if you wanted to wash your hair every 3 days, are there any tricks (products that you can use), so that your hair wouldn't get greasy/oily before the 3 days is up?

Just rinse the hair with just water, i think would take a lot of dirt out of it.
With the dirt gone, so would be the smell?
The greasiness i don’t know.

I used to put baby powder in my hair between washings to keep it fresh smelling and non-greasy looking.

Avoid places with really bad stenches, like a hibachi restaurant. Greasy food places tend to make your hair smell bad.
Don’t over-brush the hair.
Get a satin pillow case. It helps keep your hair shiny, helps keep it from breaking, and your head/scalp will also sweat less which means less greasiness.
Also, wear the hair up at night. When your hair is down it touches your face and other things which can cause it to get greasy.
Try not to touch the hair more than you need to. That’ll definitely help a bit.

Hair is supposed to have a healthy layer of oil. Chemically scraping it away causes your skin to do its best to replace it, and you end up producing too much oil. If you can stand a period of extra-oiliness, your skin should produce less and less while it rebalances itself.

It’s disgusting what the cosmetics industry has done to people’s perceptions of hair. Trying to rip away all the oil and replace it with polymers—just sad.

Yep, what @chyna says. Sprinkle baby powder over the oily areas. Then brush through while blowing hair with the dryer to get the white out. Works really well. I do this on the 3rd day after straightening my hair, too. It was a tip from my stylist.

What about dry shampoos?

Buy one of the Nature’s Gate spray deodorants (you can get it at Whole Foods) and spray it onto your scalp at night. The amount of alcohol will level out the oil, and the essential oils will make you smell good.

*Disclaimer: I have not tried this myself, but it should work in theory.

I am part Cherokee Indian and there is nothing that will keep the grease away for three days. only soap and water will do the trick each day. you would be surprised at how many people can smell a persons hair that is dirty. it really reaks on some people. soap and water, soap and water.

I heard Bumble and Bumble dry shampoo is great.

She could try doing an apple cider vinegar rise before the 3 days. You can also add herbs or essential oils to the rinse. Google it, a bunch of stuff should come up. The rinse is supposed to wash away residue without stripping the hair.

She could also switch to a natural bar shampoo with oils that will clean but not strip the hair. If she uses that, along with the rinse, she may not even have to condition. Or she could use oils to condition. Coconut, avocado, jojoba oils are all good for the hair, among others.