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They sell special heating mats for starting seedlings or you can try using a heating blanket. Just be careful with a heating blanket, as it may get too warm. Keep it on its lowest setting and put a towel between the heating blanket and the pot.

A balanced fertilizer like 10-10-10 or 20-20-20 will be fine. Follow the directions on the package when applying, simply because different manufactures have different application directions. Most home and garden centers will carry them. If possible, find a citrus fertilizer (you should be able to find them at garden nurseries) which will also make sure that micro nutrients are supplied. This article will help: https://www.gardeningknowhow.com/edible/fruits/citrus/fertilizing-citrus-trees-best-practices-for-citrus-fertilizing.htm

This article will help you:
https://www.gardeningknowhow.com/edible/fruits/citrus/fertilizing-citrus-trees-best-practices-for-citrus-fertilizing.htm

As long as you are careful with the braiding, the trees should respond as normal in flowering and fruiting once they are mature enough. You will need a rather large pot eventually to accommodate the three trees in the same pot, but for now, they should be placed in a pot that is 2 inches wider than their combined root ball. They will need to be re-potted to larger pots as they grow. I would use a slow release fertilizer every other month in the summer and once every 3 months in the winter.

Most prefer well-drained, slightly acidic soil.

I would first try soaking them in water to rehydrate them after their winter dormancy. Simply place them in the shower or gently hose them down outdoors (on mist) and allow the water to run through. Unless roots are coming from the pots, they probably don't need repotting, but you can refresh the soil by gently working the top of the soil and adding new soil in with it. If their growth seems to be impeded or they just look unhealthy as they come out of dormancy, you can then go ahead and repot them, moving up 2 sizes.