t Auto's, 3 weeks into grow...should i give nutes?

Hey everyone.

I have 2 autos (speedy gonzales and NYC diesel) and they are 3 weeks old from seed. The NYC diesel just showed its pistils 2 days ago. The speedy gonzales hasnt yet, and it is much taller than the NYC.
I have them in a pro-mix/perlite/worm compost/peat mix and water them only when very dry. They are doing well, but the NYC is pretty short, only like 9 inches, the speedy gonzales is about a foot already.
My questions are;
Should i give them nutrients? They are starting to yellow slightly so i think they are hungry.
What should my water be PH'd to. My tap water is 7.6-7.8, i PH the water to 6.6-6.8 currently.
How much do they grow after they show their first pistils approx? double in size? 25% increase?

For fertilizer I use General Organics Bio Thrive Grow 4-3-3, and the Bloom is 3-4-4.
I am picking up some CalMag this week. My other plants need it bad. I should have listened to the guy at the hydro store when he said to get it with the nutes lol.

Any help would be great and advice in general on the autos is always good. Im interested to see how the smoke is compared to non auto, and compared to 'street' weed.
Peace

Yeah start them on a light feed.
I ph to about 6.5 in promix.

Alright, cool. Ill PH some water and start them on 1/4 strength nutes.
Should i use the veg or bloom? Im thinking veg for the first couple feedings, since they are needing nitrogen, then switch to bloom nutes.