Do you mean geographic coordinates?
At any rate... Your eTrex 10 should accept the geographical coordinates needed to find a cache. Those final stage coordinates format should appear identical to those of other stages, aside from the specific digits.
In the northern part of the western hemisphere, they would appear as: N XX xx.xxx W XXX xx.xxx. This is Decimal Minute Format.
How is it that you obtained the third stage coordinates? Given... as found in the second stage; or computed yourself by stage clues?
Showing us a sample of the coordinates (format, or layout) would go a long way in helping to answer the question.
I looked at your geocaching Finds, and see http://www.geocaching.com/geocache/GC72B_pine-trailhead-multi-cache , but that is only a two-stage cache.
You really should find the final stage before marking it as "Found".
Ok, we HAVE NOT logged the multi that we are working on which is a four stage Lonelycache.
I'm not sure if they're Celestial coords, but the coords look like the examples given.
Here's a example:
00* 00'0.00"N
00* 00'0.00"W