Give it a half hour to dissolve and mix in and then let them swim. Even though the bag says 3ppm and under is safe, that is not the whole story. Needed chlorine levels are based on CYA levels. The higher your CYA is, the higher your chlorine needs to be. So, if you had a high CYA level and kept your chlorine at 3ppm, it would not be safe nor sanitized because your chlorine level would be too low.

Take a look at the Best Guess Chlorine chart in either my signature below or at this link: http://pool9.net/cl-cya/
It explains the relationship between CYA and how high you must keep your chlorine. The fact that many people do not understand this connection causes lots of green pools!

You are losing chlorine fast now because you don't have any CYA in the water. Dichlor adds CYA as well as chlorine so after you use it for awhile, your CYA level will build up and you'll be able to maintain a chlorine reading throughout the day. When your CYA gets to around 50ppm, you'll want to quit using dichlor and switch to just bleach so your CYA level doesn't get too high.