So far, so good. I already replaced the battery in the watch, and it works and fits perfectly.
The battery appears to be the correct version for the TomTom Runner/Spark 3 but there was no print on front/back that said so and no information came with the package. The battery on the supplier's website has printed text. The shipped battery had nothing, as you can see in the attached picture. This means it is risky to open your precious watch and replacing the original battery by an unmarked brand. Positive note is that it does appear to do a good job and has identical connector as the original Runner 3 battery. I would have given more stars if it weren't for the fact that I bought another Runner 3 battery from another supplier, for the same price, which arrived 10 days faster, had clear printing that it was a battery for the Tomtom Runner/Spark 3, and included a set of replacement tools.