RAID10 needs at least 4 disks of same size and preferably same speed.
There is a lot of debate whether to go RAID5 or RAID10. RAID10 is better at writing speed but RAID5 is marginally better at reading. RAID10 can suffer a two drive failure wheras a 3 disk RAID5 can only suffer a single disk failure.
Personally I would go RAID5 and use the 3 x 1TB drives you already own.
The 500GB drive can be put in for storing stuff you don't care about and don't want on your RAID setup