ipnat & load-balancing outgoing traffic

Hi,

I have the following setup:

Internet ---- ext_if1 - [ ]
Internet ---- ext_if2 - [ NetBSD NAT router ] - int_if ---- Intranet
Internet ---- ext_if3 - [ ]

ext_if1: 123.456.789.1/24
ext_if2: 123.456.789.2/24
ext_if3: 123.456.789.3/24
int_if: 10.0.0.1/24

My /etc/ipnat.conf looks like this:
----begin-quote----
map ext_if1 10.0.0.0/24 -> 123.456.789.1/32 proxy port ftp ftp/tcp
map ext_if1 10.0.0.0/24 -> 123.456.789.1/32 portmap tcp/udp 1025:65000
map ext_if1 10.0.0.0/24 -> 123.456.789.1/32
----end-quote----

I.e. I am only using Internet line #1. I would however like to
load-balance all outgoing traffic over all 3 Internet lines. How would I
do this? Could I simply add a
----begin-quote----
rdr ext_if1 123.456.789.1/32 -> 123.456.789.1,123.456.789.2,123.456.789.3
round-robin
----end-quote----
after the aforementioned 3 lines, or would this confuse ipnat?

TIA,
- Daniel