miltiadis_s, as per your PM, i'll put some instructions to point you in the right direction, but i'll post em here so other members can see when they check this thread out incase they wondering the same thing.
Ok, firstly, running three small neons won't overload the intended power. Also, keep in mind, that your boot light shares the same fuse as the brake lights. You could always beef up that fuse if it blows out. (Check you don't have a short circuit first though) I'm running 3 neons in parallel (More amps to run) and the factory fuse has not blown, and the factory switch handles it just fine. and i tested that the fuse doesn't blow when brake lights are on too which means i'm pulling max power on that fuse. (If you run them in series, it uses less current, but they run more dull, and being neons, they may not light properly)
Second, be sure to keep the controllers that the neons are connected to. These act as regulators for the neons (most types, some are happy to just run stand alone...), and help prevent voltage spikes, also i can open the boot and easily switch the lights off if i need the boot open for an extended period and don't need them on.
Thirdly, as far as connections. Using the factory two wires, your options are: (using an example of using 3 as you intend. Please xcuse my crud paint diagrams...

)
OPTION 1 Parallel:
Each positive wire from neons, connect to the same positive wire as boot light normally would. (IE: Factory positive wire has 3 wires running to the neons. also keep in mind that the POSITIVE is the switched wire for your boot light)
Each negative wire from neons, connect to same negative wire as boot light normally would.
OPTION 2 Series:
You will "daisy chain" the neons... Positive from factory boot light to positive on neon 1. negative on neon 1 to positive on neon 2. Negative on neon 2 to positive on neon 3. Negative on neon 3 to negative on factory boot light negative.
Good luck, hope this helps...