Raid 0 on 3x500GB triples your failure rate (especially important on older drives, as I presume these are), and still won’t get anywhere near an SSD in speed.
You could just mount the 3 drives separately and have storage that way, which means if one fails you’ve still got the data on the other two… it’d still suck but not as bad as losing everything.
If it was me I’d wait until I could afford the SSD… it’ll be many times faster and newer.