I was a maintenance mechanic for many years. I am licensed and certified in many areas of construction. What I found when flipping houses (in which I have flipped many) is that the technical trades are the ones you don't want to get too deep into. (I will explain why later). There are many that do such. The hardest trade to find, believe this or not, are great painters. Good painters are hard to find. I am talking about the reasonably priced, dependable, correct and quick ones. Many people will tell you they are good painters, when they are not.
Now lets get down to the most costly people in flipping. These are the under-house - in attic people. I don't know about you, but I am someone that has never enjoyed being under a house in a crawl space or in an attic. These people are normally expensive as well. Included in this category are duct installers and insulators.
At present I am flipping a home in a small town. Something that this small town does not have is carpet installers. It has been frustrating to say the least. Matter of fact I am talking with the painting company I am using (which consist of two women) to add carpet installation to their business. Matter of fact, I may finance them to do so.
Now many states and areas have different regulations for codes. Some do not allow landlords to do their own repairs period, without pulling a permit, and you must have a licensed trades person to do the job. Water heaters come to mind.
So, if you are going to flip a house to rent, I suggest that you buy the property, and start with all of your electrical and have a good licensed electrician to check it out - remove fuse boxes and replace them with breaker panels. Replace anything he thinks may be a problem in the near future. Replace the plumbing if need be. Pex tubing and ABS is my advice. If you have the basic knowledge of both it should be simple for you. If the HVAC is older than 5 years, replace that as well. Old water heater replacement is a must. I will also tell you this. Most people that let their house go into foreclosure is for 2 reasons. They are, the HVAC system, and roof. Both of these are very costly to the inexperienced people. (Unless you want to do roofing, that is costly to everyone).
You asked what trades to focus on, believe this or not, they are the most common which you have experience in. Getting a license to be an electrician, plumber or HVAC mechanic takes time, and you must keep up with the changing codes on a daily basis. If you are a master in either of these trades, you are better off starting your own business in those trades and flip properties on the side. Basic plumbing skills is a must for anyone flipping a property - I will go further on this, if you don't know how to do basic plumbing, like how to run water and drain lines, or how to replace a water heater to code, maybe you shouldn't flip a home to begin with - these are easy to learn and they can be costly if you have to pay someone to do such. The same goes for light receptacles and ceiling fixtures.
Now if I was to start a company, it would be in basic home repair to consist of painting, drywall, ceramic tile (wall and floor tiling) floor sanding, counter-top replacement and carpet installation. If you know these trades along with basic plumbing you should be good to go. I don't know of anywhere that it requires a license trade mechanic to do fixture replacement. Fixtures are just that, however. They are light fixtures and sink fixtures. Now think about this paragraph alone. What are the two most expensive places in a home to renovate? If you know these trades, you can do both very cheaply, if you don't need to run or move plumbing and electrical runs. (Even if you have to do this, you can run the lines, and have a licensed trades-person to make the final connections).
My advice to anyone and everyone that flips properties full time, or has rental properties. If you are going to pursue a trade, do that first, and flip on the side. If you want to build a portfolio, or flip homes to sell, do that. A real estate license would be your best bet to pursue, and you need to work in it to do so. Knowing the basics in trades is what you really need to know along with an active brokers license. - That will save you the most money in the long run in any real estate investment.