This is my 3rd grow but now im ready!?

But I don't know if my post went through,I have 5 plants (3 in soil 2 in a bubbler)I have a 4o0 wat mh .the light is 20 in from plant # 2 but the rest of the plants are about 12 in away.I really want to do good with this.This is my 3rd time trying to grow.The first was under an incadescent,really tall and lanky(no good)the 2nd time I grew and flowered with flouros,I harvested about a blunt.This time it's the metal halide.The other day I had the plants in an enclosed space and Plant #3 was burned,so I had to clip parts.The plants are about 3 weeks old.
Note:I am also wondering if I started feeding too soon,becuase the plants #1 and 4 and really green almost like a dark green.plant #2 and 5 are in the bubbler,I guess plant #5 is the reason you shouldn't put two plants on one pot.The bubbler was started today.

As you can see plant # 5 is very little.Also what is the proper distance for the plants under the 400 watt bulb.I don't want to burn them

How much can 5 plants yield?

I made a mistake,plants 1 and 5 are in the "bubbler" sorry.This is what I had them in,a hollowed out dresser but the metal halide got it too hot.they are just sitting out in the open now

I don't know how much 5 plants yield but they haven't been sexed yet so I hope I get at least 1 female.Also,what's wrong with plant#3(the one in the styrofoam cup) the bottom leaves are yellow but it also looks like a bleach spot on those leaves any suggestions,also I was wondering if my bubbler has a ph of 6.5 can I lower it by adding a lemon peel(to make it more acidic) until I get the money for "ph down"

I think 12" from your light is too close considering the size of them, only put them that close after about 4 weeks assuming everything is ok. When did you start feeding? I'd start around the 2-3 week mark. You should get them in bigger pots now, they could well be root bound. Hope this helps, good luck!
Oh and yield for 5 plants depends on strain and growing conditions, indoors i'd hope for around half a pound.

If you want good yield from those plants you will need to purchase a MH to HPS conversion bulb. Make sure its a bulb compatible with a MH system that is HPS not vice versa as to not short something out. I think 400W one of those would be around 50bucks or so, maybe a bit more.

In case you were wandering you can flower under MH light but it is not recomended for higher yield although it is possible and some people do it.