"Does synthetic compounded T3T3/T4 have to be made as timed-release? "
- No it does not have to be.
But if you take a synthetic med with T3, you might experience peak and 'crash' of energy levels. Unless you decide to take two compounded capsules a day ($$). That is why most who take cytomel split the dose AM and afternoon.
Also, one would want to take the T3 as a time release anyway, as you want to take it spaced out over the day, not just all at once as it has a short half-life.
The only synthetic T3, T4 product is Liotrix, sold under the brand name Thyrolar. If the drug is synthetic, then it is available via a regular, non-compounding pharmacy. As far as I know, it is not timed-release but has a set amount of T4 and T3. This can be a disadvantage compared to taking T3 and T4 separately since you can't adjust the dose of either.