Hi Celine, thanks for reaching out! I can point you in the right direction for sure. This is a good question, because it's more complicated than it should be honestly. I'm not sure which of these ways you will prefer, so I will just list them all just in case.
Some may require you to download a particular software and make an account (Steam for example) and in most cases, getting the full version of Bejeweled 3 on PC will be paid software rather than free, but these are all legitimate services and platforms at least.
EA's Website offers the game, but it seems to be a part of a subscription service? I'm not really sure about the details, and I personally wouldn't recommend this approach unless you were a big fan of other EA games, but here is the link anyway: https://www.ea.com/games/bejeweled/bejeweled-3
You can make a free account and download Valve's Steam gaming platform, which is a popular storefront for many digital PC games. They have Bejeweled 3 for $4.99 at a normal price, but Steam games often go on sale multiple times a year, so you can wait for that price to come down if you wish. This is how I play the game, and is the method I would recommend for most PC players if I had to pick one. The Steam store page for the game is here: https://store.steampowered.com/app/78000/Bejeweled_3/
The MSN Games Zone website used to have a version of it that you could play on the web for free, but since the death of the shockwave flash format, it's no longer available to play there.
Like with Steam, EA has their own specific game store/software called Origin, and they also list the game there for $4.99. Between the two I would recommend Steam simply because it is a much larger, more inclusive platform for thousands of other games, vs. Origin mostly being all about EA games and their ecosystem, but that's just my preference. https://www.origin.com/usa/en-us/store/bejeweled/bejeweled-3
You can buy a physical version of the PC/Mac release from online retailers like Amazon, but the price goes up a good bit compared to the digital versions. I'm seeing copies on Amazon going for around $20-$35 USD currently in various conditions here: https://smile.amazon.com/Bejeweled-3-PC/dp/B0047SIUZI
If I'm reading things right, it looks like there's a free trial version you can get on CNet. I'm not sure if the trial version lets you play in Zen Mode or not, but I'm fairly sure you would be able to upgrade to the full version by purchasing it from within the trial version. I haven't installed this version of the game myself, so I can't say for sure if it's a good way to go about it or not, but the link to it is here if you wish to try this approach: https://download.cnet.com/Bejeweled-3/3000-2111_4-75329716.html
Finally, I know you asked about Bejeweled 3 for PC, but for the sake of thoroughness, I'll just mention that if you have a smartphone or a tablet at your disposal, you can download Bejeweled Classic for free, which is for all intents and purposes the same game. It's on the Google Play Store for Android devices here: https://play.google.com/store/apps/details?id=com.ea.gp.bej3&hl=en_US&gl=US
and on the Apple App Store for iphones and ipads here: https://apps.apple.com/us/app/bejeweled-classic/id479536744
This... was probably long enough to be an article in and of itself, but hopefully it's helpful for you and any others who have the same question.