He is telling you to keep them all three in veg but stack them so the shorter ones get more light and therefor grow faster. Then after the shortest one is at least 12" tall, change them to flower/bud cycle. You will get NO product from that shortest one if you put it into flower now. Also, you have to consider age as well as size for agiven variety when deciding to move them into flower.
Regardless of what cycle they are in, if you only have one light you need to stack them so that the tops are all about the same distance from the light. If you put them all on the same shelf or floor, then you will either light starve the shortest one or burn the tallest one. Each plant MUST have the light source distance adjusted to meet ITS OWN needs either by positioning the light or moving the plant closer or farther apart.
Why in the hell would even a first timer stagger the seeding/cloning of multiple plants of the same strain if you only have one grow area? God, my 8 year old grandson knows that if one plant is 2 weeks older than the other, they are NOT going to bloom at the same time or develop bud/fruit at the same time.
If this sounds like too much trouble, consider switching to the auto flowering varieties like lowryder crosses that veg for 2 weeks and then go into flower regardless of the light cycle. All you have to do with them is plant them water them, feed them the proper nutes for their cycle and harvest them. They do NOT need the light cycle changed or changed from MH to HPS, although using a combination can increase your yield, in fact cutting the lights back from at least 18/6 will probably decrease your yield.
The neat thing about them is they finish in around 70 days from planting the seeds to harvest and you can stagger your crop because they all use the same lighting. All you have to do is to feed them veg nutes for the first 2 weeks, then switch over to flower nutes and finally to water just before harvest.
If you need more information, I believe there is a thread on auto flowering on this forum. If not let me know and I can get you a link.
Also, the auto flowering are very light sensitive -- they are short and bushy so you need a lot of light.
Hopefully this will hlep.