When you make the RAID layer above it, do it on the LVM, so you will save a lot of trouble if you set not very well estimated partition sizes.. The idea is that if you don't use XFS or ZFS or some other FS that allows resizing partitions like EXT2 / 3/4 for example things become a big hassle when you realize, that you didn't do the most- good division. In general, you get maximum plasticity if you need to reduce or increase the size of the partition and at the same time you are protected against unpleasant events of your data. In general, something of this kind is obtained

| / | /var | /usr | /home  |
 --------------------------
|       LVM Volume         |
 --------------------------
|       RAID Volume        |
 --------------------------
| Disk 1 | Disk 2 | Disk 3 |