This can cause pressure to you man. But if you have a budget and aiming for the latest gadget, then by all means go and buy. But your Samsung Gear S3 is still doing good so for practicality reasons, you should stick with it for a while. That is also to use the full potential of your Samsung Gear S3. It depends on your need man will it be for the upgrade or for the function. And Samsung gadgets have good quality and their gadgets last for years. So for me its best to stick with your Samsung Gear S3 and run more miles have more activities and workouts or marathon perhaps

Enjoy Samsung Gear S3
I agree with this. As long as what you have still works, I see no need to replace it. However, if the most important function to you suddenly doesn't work anymore, like showing your pulse, and nobody can help fix it, then you could consider buying the latest thing.
Like everything else in commerce, everything new gets priced really high, because everybody wants it, and over time it'll drop in price, or goes on sale. If it were to go on sale and your Gear S3 still works fine, you could consider it.
It's just a personal habit to never buy things at the high price, and not be persuaded into buying every new thing. This does come at the downside that I'm always buying a smartphone that has been released two years ago.
It's your choice really, but I would recommend waiting for your watch to stop working the way you want it to work, or until the Galaxy Watch goes on sale or drops in value. OR, try selling your Gear S3 at a decent price, so that what you need less money to buy the Galaxy Watch.