Symbol 3 means that the loco will accept an LGB 55020/55021 decoder (or a Massoth L, which is essentially the same except cheaper!) plugged directly into the loco's main board, with no wiring necessary - in fact you normally cut the four track and motor wires off the decoder when you plug it in - a number of pins on the underside of the decoder mate with sockets on the loco board so that the two boards are linked piggy-back.
The newer 55027 decoder, which comes with a 10-wire loom and plug as Mark mentioned above, is only plug-and-play into a loco with symbol 5, not symbol 3.
As to the other symbols, just in case others are wondering, 1 (the embossed "D" in the gearbox bottom plate) simply means the gearbox is a 4-pin type and thus can have a decoder fitted without internal re-wiring of the motor; any gearbox without the raised D will be a 3-pin (or even a much earlier clamshell type) and will thus need some fiddling with before it is decoder-compatible - it's not a difficult job by any means, but just another thing to do.
Symbol 2 means that a decoder (again they were referring to the 55020/55021/Massoth L, but other types can be used) can be fitted using an LGB/Massoth 6-pin interface cable and plug, available as an extra.
Symbol 4 means that the loco is already factory-fitted with a DCC decoder.
Symbol 5 meansa the loco has a 10-way plug for direct installation of an LGB 55027 or any other decoder (eg: Massoth XL/XLS) which is fitted with a 10-pin loom and plug.
Symbol 6, I believe, indicates and interface for the direct plug-in of one of the new LGB MFX multiprotocol decoders, or an equivalent such as the Massoth XLS-M1.
The un-numbered symbol in the photo is a variation of symbol 4, and means the same - a factory-installed decoder.
Note that when installing any decoder into a loco with symbols 2 or 3 (and possibly 5?), there will usually be a bank of dip-switches on the loco board which MUST be flipped to "off" when the decoder is installed - failing to do this can fry decoders, and you really don't want to let the magic smoke out....
Jon.