You want to do an RDM (Raw Device Mapping) to an internal disk, which is not supported (as KB VMware KB: Creating Raw Device Mapping (RDM) is not supported for local storage )
But you can make an internal disk as RDM as explained on this blog post from David Warburton http://blog.davidwarburton.net/2010/10/25/rdm-mapping-of-local-sata-storage-for-esxi/
Also, you will not be able to share this ext3 disk with more than one VM at the same time, if this is what you want, but this is due to a ext3 limitation (you will need a clustered filesystem like ocfs2 or gfs).