Usually 10 days before harvest is when you start flushing.

Yes, it is fine to flush with plain pH'd water. Some people use a flushing additive like clearex or what have you. i've never used anything besides water, i've been meaning to look into a product.

you say you are at 3 weeks flowering? and you plan on harvesting in 2 weeks? so you are only planning on flowering for 5 weeks total? 99.99% of plants take AT LEAST 8 weeks to fully mature. some strains can be done in 7 weeks...but there aren't many.

I would say be sure to check your trichomes with a 10x-30x magnifier and make sure they are, at the very least, all cloudy. It all depends on your personal preference, but i think most people usually harvest when it's like 80% cloudy and 20% amber. i like 90% cloudy and 10% amber.

just remember:
Clear = not mature
Cloudy = Peak
Amer = past peak, THC starting to degrade.

Here's a link to Zandor's flushing methods. Pretty much industry standard. http://boards.cannabis.com/grow-faqs...ml#post1254412

I would give them a few more weeks than just 5, most of the time, if everything is going well, you'll start to see massive bud growth around week 6 and on.

good luck
Skeet