Adding the Propionyl-CoA Pathway to E.coli
If I understand it correctly the pathway goes: Acetyl-CoA --> Propionate --> Proprionyl-CoA --> ... --> 3-HP


Not exactly. The first reaction is:
$propionate + acetyl\textrm-CoA \rightarrow propionyl\textrm-CoA + acetate$
The CoA group is transferred from acetate to propionate. The following reactions are an oxidation to acryloyl-CoA and a subsequent hydration to 3-HP-CoA. Lastly the CoA group is transferred back to acetate.

Propionate is a 3 carbon molecule and so is 3-HP. So stoechiometric coefficients of 1 seem logical.