I use Illustrator and it's something I'm still not very good at. I'll take, say, a 24×24 shield and then horizontally stretch it out to around 30''. Naturally, this creates the "bubble" look, so I then edit a few of the anchor points so they maintain the same angles as before.
But even then, the look is still off. For example, I've tried to create a "wide" 1961-spec US Route shield (of the black square variation) and it just doesn't look quite right.
The short answer is No. There is no general way to create a three-digit shield by stretching the two-digit version. Typically, you have to identify what elements of the two-digit shield are "repeated" in the three-digit shield and then figure out how to regenerate them within the added width. For example, the Kansas sunflower shield consists of two basic petal elements (one which points straight out and the other of which has a "hook" pointing clockwise) and these elements are scribed around an ellipse to produce both the two- and three-digit shields. In the case of the two-digit shield, the ellipse is a circle.
My usual procedure for creating three-digit shields, in states where the marker shape permits it, is to saw the two-digit shield in half and align the two halves respectively to the left and right sides of a rectangle that has the correct outer dimensions of the three-digit shield "envelope." Then I weld in a rectangle to fill up the gap between the two pieces. This works well for the generic three-digit rectangle markers used in states like Massachusetts and Rhode Island, the old pill-shaped three-digit marker that was used in Oklahoma before the advent of the meat-cleaver markers, the Pennsylvania keystone, and some others. It doesn't work for the Kansas sunflower, California miner's spade, and plenty of others. I think it might work for some variants of the US route marker--but only to a point; you would have to re-do the double scroll at the top since that is in effect "stretched," and you would have to do the same for the double scroll at the bottom unless there is some tangent length on both sides as specific provision for stretching. (California used to have a specification for guide-sign US route marker which was designed with tangents at the bottom to accommodate easy resizing to multiple widths. This was abandoned when the outline-shield era came to a close.)
Simple stretching works for some states (e.g., ones like New Mexico where the three-digit guide sign route marker is an ellipse) but not for others.