plant is getting too tall

got 3 plants under a 600w hps, got light bout 2ft away from from tallest plant but its getting taller and taller, its 3 weeks into flowering.will it stop soon or is there some way of training it some how?

There are several ways to train the plant:

You can tie down the colas - that's called low stress training - Lst

You can bend the stem until it permanently leans over - that's called super cropping. You can search either of these terms on this site for more details

can supercropping be donewhen flowering then?

You can super crop anytime.