EXPERT RESPONSE
So if I understand your question correctly, you want to know how to create one half of a mirror out of three drives that total 300 GB, and the other half out of one drive that's 300 GB. Well, whether you can actually do this will be a function of the RAID controller. Typically, intelligent RAID storage controllers do not allow you to do such a thing and usually reply with some form of "huh?" error message.
This trails into your second question of what type of read/write performance you would get. Just the fact that you're asking the question implies that you probably already know the answer: "Who the heck knows!" You'll probably get something on the order of 70 to 210 IOPS and somewhere between 10 and 30 MBps, making the array as predictable as next week's weather.
RAID best practices has always stated that identical disk drives are the way to go: same sized drives, same spindle speeds and symmetrical number of drives (no three on one side and one on the other type stuff). However, if you're not using it for mission-critical work and/or performance is simply not a concern and/or the controller allows you to do this and/or you're the type who liked to throw grasshoppers and spiders together in the same zip-lock bag just to see what would happen, then go for it! Just don't expect what you would expect if you set it up like your proposed (and correctly proposed, I might add) RAID 5 array.
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