I recommend an OSSC such as this (first hit in Google, so not recommending any specific model or retailer). This will work with all your consoles and is far cheaper than getting individual HDMI adapters for each console.
The worlds best zero lag retro gaming upscaler and video converter is now even better.
videogameperfection.com
Loads of used options on Ebay too.
Then I would use a high quality scart block so all consoles can be plugged in at once - Note that this is 100% optional. I have an auto switching one from Otaku Games (website seems to be dead?) but it only detects RGB enabled consoles, so the NES and N64 were no good. Make sure you get yourself a decent scart cable to go between the scart block and the OSSC too. You can get away with £5 scart cables for each console to the scart block - Yes it is not as good, but at £25 per fancy scart cable the price gets silly.
Whatever you do, avoid a cheap upscalers as they are awful.
Also avoid cheap scart switches - They have poor picture quality and your speakers will really hiss.
Using an LCD/Plasma tele with consoles plugged into the scart socket is a big no. The picture will be appalling since the TV doesn't know what to do with it. I am pretty sure the OSSC can deinterlace too which helps the OG Xbox and PS2 since they typically outputted 480i. Your PS3 will be pretty much fine plugged right into the TV HDMI port. The OSSC is not cheap, but it will be great value. I don't have one myself as I use an old CRT tele, but >25" non-widescreen ones these days are rare and pricey. Keep in mind for light gun games you will need a CRT (no idea about the Wii though, never owned one).
As I am a massive retro nerd, keep an eye out for the whole NES/SNES/Megadrive/PS1/PS2/N64 PAL vs NTSC argument as often the PAL version was sluggish and the picture was squished vs NTSC. A common example is Sonic 1, but not all games can be forced into 60hz mode.
The OG Xbox, GC and Dreamcast would ask upon bootup if you wanted 50hz or 60hz, which meant the PAL vs NTSC argument was void, but the PS2 never seemed to.
There are lots of hard and soft mods to get a better picture or force 60hz on a range of consoles, but console modding chat is not for these forums

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I have eddted this response more times than I can count!
Please post back with pictures of whatever route you pick

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