Multikey Index GISt

I need to implement a new GisT index that use multikey (i.e. tree columns).

I need to storage in the entry index the three transformed columns
alltogether.

If a use "create index anIndex on tablename using ( atr1 atr1_ops, atr2
atr2_ops, atr3 atr3_ops) ",

I will obtain three separated invocation of "compress" function (one for
each XXX_ops). I need to receive them in only one invocation. How can I do
it using GIsT?

I am using PostgreSQL8.2.3. How does it work the multikey GIsT announced in
the latest version of PosgreSQL?

Thanks a lot,

Leticia