I think the real answer would actually be "it depends"!
Very often, in case of remote/DR sites, Simpana will be able to go faster than what your network link will support. In that case, have a multiple to one relation is most likely fine.
If you have a dark fiber then it might be right that the MA will not be able to go as fast as the dark fiber thus providing slower throughput than what 3 MAs at your DR site would provide.
I think the real question would be: is the throughput of a multiple to one MA copy good enough for your DR requirements? Often, we'll want to have all backups done ASAP to free up our production servers and complete backups within the backup window given to us. Copying the data offsite is another story though. Now, you don't have any production servers involved anymore... only MAs and you might not be using the same link as your enterprise uses which makes it possible to have MAs copy data over the dark fiber until 10-11am or even later in the day. This is an answer only you would know.
Since adding MAs is pretty easy, you could always start with 1 MA and scale up eventually if needed. Just warn your client of that possibility so he doesn't get back to you later on because you need him to buy new hardware, etc.
One other thing you could also consider though, is keeping 1 DDB per MA for your DR data and having those DR DDBs hosted by each of your source MAs. This way, you will have 1 dedicated MA per DDB and then all that your DR MA needs to do is receive the unique blocks and write them to the maglib which it should definitely be able to do easily. Obviously, this would require a bit more storage at the DR site as the data would be siloed in 3 DDBs but that might be a better solution in your case.
Last thing, in case you haven't seen it already, you might be interested by what Stephen from Commvault just posted: http://forum.commvault.com/forums/thread/21218.aspx.