Okay, you have a few options here, again really depending on your requirements. If you wanted to benefit from the HD audio offered by the PS3 in Blu-Rays, you could go for one of the latest amps like the Sony STR-DG820 (between £250 - £300), or the Yamaha DSP-AX763 (£350 from Richer Sounds VIP club at the moment). Either of these will allow you to hook up your PS3 via HDMI and benefit from HD audio and also connect your HTPC via HDMI and optical. A single HDMI out to your TV means all the video switching is done via the amp. The Wii could also be hooked in via it's component cable.
If HD audio isn't so important, you could go for one of the cheaper AV amps on offer, but there are a wealth of these. Does music fit into this anywhere? Are those the only three sources you have?