DDR3 is the same as DDR3L except for the voltage. Some DDR3 kits would be able to run at the 1.35V of DDR3L, and you may also not be worried about exceeding Intel's official maximum memory voltage of 1.35V. In either case DDR3 would work just fine.
It does require a motherboard with DDR3 slots (which are exactly the same as DDR3L slots).
Yeah this is true, what I meant when I said it wouldn't physically fit is that it wouldn't fit into a DDR4 slot. I said this because it's near impossible to find a higher end board with DDR3 slots on them. If you do get a board with DDR3 slots then male sure you set your voltages correctly or you can do some significant damage.
When a company says that something isn't supported it doesn't mean that it won't work or you can't get it to work. What it means is that they haven't tested it, will not be keeping things up to date to keep it working (bios updates etc...) and they will not warranty it if it breaks. They also will not help you out with any technical related issues that arise as a result of an unsupported configuration. You can still get things to work though but to me it isn't really worth losing warranties and risk having an unstable system for an unsupported configuration (unless I had loads of money and was testing it as an experiment, but I wouldn't keep it as a crucial work PC).
If you do decide to pair DDR3 with skylake then please get back and let me know how it went. I'd like to know what issues, if any, you came up against and how well it works.