I've read so much I've confused myself.
Where these things always get complicated is when multiple things have to work together.
Bottom drain - 3" this is going to gravity feed to a.series of three 55 gallon drums (settling and mechanically filtering. Pump will draw from 3rd drum and will split to two returns. A 4K p/h pump will be pulling through and pushing to the returns. Is this too much?
Yes, too much. 3" should really be in the 1800-2500 GPH range. Just talking pipe size here.
The next issue is pushing 4000 GPH thru a 55 gal tank. That isn't going to settle much of anything. Good rule of thumb is GPH * 10% = tank size. But this kind of depends on the performance you want. Some people are really happy if they see some dirt come out of the drum and consider that to be a working settling tank, and they think the dirt piling up in the pond can't be helped. Whatever makes a person happy, it's just a hobby. If you don't want dirt piling up inside the pond it takes some number crunching.
Hooking tanks in parallel instead of in series would reduce flow in each tank. Tricky though to balance the flow, tricky even to measure the flow. So you could hook something like 8 drums in in parallel for 4000 GPH to get reasonable settlement. Not great mine you, but reasonable.
The next issue is when you turn on the 4000 GPH pump the water level in each drum will go down. That could affect their filtering performance and being in series might even run the last drum dry. Kind of depends on what kind of media is in the drums, where pipes are placed, etc. But I'd be betting dry drum.
Skimmer (Savio filter Skimmer) with 55 watt UV light. This is going to feed my 150 gallon skippy style filter/waer fall located on the opposite side of the Skimmer. It came with a 4K p/h internal/external pump. Where do I locate the pump? It can sit in the Skimmer and push or I can put it closer to the skippy externally.
It should be as close to the skimmer as possible unless maybe if you ran a 4" line from the skimmer to the pump. Pumps do best at pushing water.
Normally UV goes after the filter so it gets cleaner water so it works better and there less chance of clogging. The basket in the skimmer catches a lot, but when you remove the basket to clean it leaves and stuff stuck behind the weir, stuck to the sides, etc., can get washed off and go into the UV and might not come out. And of course stuff is still coming into the skimmer while the basket is out.
I am all planning a 6x 12 bog/veggie filter that is elevated and well gravity feed back into the main pond. How much flow for the bog? Can I take water from the return or Skimmer and send to the bog or do I need a seperate pump?
Bogs can take as much flow as you like, meaning there no reduction in bio filtering at higher flows. Higher flow means more food, O2, carbon getting to the bacteria.
Bogs also act as mechanical filters, basically the same as a settling tank. Because water has to move thru the gravel it acts as speed bumps, limiting the speed no matter the pump size. Pushing 1,000,000 GPH thru a bog would still act as a settlement tank...but only for the water actually going thru the gravel. At high enough flow the water will back up and you'll see water on top of the gravel. Water moving across the top of the gravel wouldn't be very good mechanical or bio filtering unless macro algae starts growing there and then it's great (basically a stream). Flow can be so great that the water backs up enough to overflow the bog entirely. Adjusting the amount of gravel normally fixes both issues.
I think I understand what you're saying by "bog/veggie"...you mean a bog with some plants. That would really be just a bog. If you're counting on the benefits of a real veggie filter you will probably be disappointed. Initially, if plants are planted bare root, the bog will act as a veggie filter. As muck collects it will be less and less of a veggie filter and start producing more and more of the elements a veggie filter would remove. Not saying bogs aren't good, just they do what they do and that's it. Unless of course you clean the bog a couple of times a year which is a petty huge pain imo.