With one "th" sound immediately following the other, everyone would just make a longer "th" sound and slide into the next word (something like, "withhhree"). However, the "s" is a distinct sound, so it really can't be merged with the preceding "th." Therefore, you'd make a pause and a separate sound between the two words.

I hope nobody heard me sitting here trying to work out how I say it... It would have sounded rather odd. But after several sayings, I sort-of concur with Brad. I think however that your tongue does tend to push tighter on your teeth in the middle of the long thhh, so it's not quite continuous; you do slightly re-voice the second th as well.