3mm is pretty dang close, I think that size is more suited for wallets and the like. here's an excerpt from a post that I showed a sheath that I instantly regretted using 3mm, it was by accident but once started...had to complete
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I have a few but seem to always fall back on my quite old but still serviceable Craftool 8042 which is a 3/32" prong, it provides enough room for me to use the saddle stitch with the two needle method that I am growing to like a lot.
I picked up some 3mm and 4mm and have used both, the 3mm by mistake and boy that seemed really really tight stitching, here's a photo showing a spacing comparison, the sheath at the top used the Craftool and 1.2mm tiger thread
Untitled by
GaryWGraley, on Flickr
Doubling back at the beginning and ending of the stitching to keep those points tied down tightly
Untitled by
GaryWGraley, on Flickr
This sheath I started to hammer in the stitching holes and "Thought" I was using the 4mm but after I hammered in two sets I thought wow, that seems really close together! and examined the tool closer and saw it was the 3mm

but...I had already put so much time into this sheath I had to continue and it was a LOT of sewing through some very thick leather!
Untitled by
GaryWGraley, on Flickr
Untitled by
GaryWGraley, on Flickr
BOTH boxes had the same photo of 4mm set on them...of course lol
Untitled by
GaryWGraley, on Flickr
Untitled by
GaryWGraley, on Flickr
So, depends on what brand I think you pick, but the ones I had that said 3mm were too close for me to use.
G2