Hello Rolando,
if that router is outside the OSPF domain, one router in area 3 will act as an ASBR (autonomous system border router) by injecting in the OSPF domain the routes that are learned or point to that external router.
For communication between the ASBR node and the external router you can use:
static routes
a BGP session
or another IGP to build a neighborship in a common link (including a different OSPF process or RIP or EIGRP or ISIS)
So depending on your choices or the external router simply has a static route for the IP address block that spans over multiple OSPF areas (behind area range) or if a dynamic protocol is used it is the ASBR node that has to advertise this block over the session with the external router.
A typical trick used in this scenarios is the use of a floating static route to null0 on ASBR:
in order to have the desired IP prefix in the ASBR IP routing table.
example
ip route 10.128.0.0 255.254.0.0 null0 250
this allows then to have the prefix ready to be advertised in BGP or other routing protocol using the appropriate network command
Warning:
null0 is a waste bin that is used to silently discard packets so it is important that:
you use an high AD value and that more specific routes exist in OSPF domain in normal conditions (no faults)
Hope to help
Giuseppe
