All three of these are global phones, more or less. The OnePlus 5 is covering most of Europe, the US and Canada, India, and China at this point. The Galaxy S8 and iPhone 7 Plus can be had pretty much anywhere.
The OP5 is the most affordable of the bunch - that's one of its chief claims to fame. At $479 for the 6GB/64GB version, it's about $140 cheaper than the S8 at its current price in the US ($100 off MSRP). The gap is similar in Europe. There's a case to be made for spending the $60/€60 for the 128GB storage version of the OP5 since it's not expandable, and if you follow that thought, the S8 might make even more sense.
Comparisons to the iPhone 7 Plus are a lot easier on the OnePlus 5. Since the iPhone doesn't come in a 64GB trim, and the OP5 in turn isn't available with 32GB of storage, the 128GB iPhone 7 Plus comes at a $330/€450 premium over the OP5 - that's a lot of money. Even a 32GB 7 Plus vs. a 128GB OP5 means a $230/€340 advantage for OnePlus.
All of this is just the raw numbers without taking into account the retail bundle. The OP5 ships with just the charger and a USB cable - no headphones. Both the iPhone 7 Plus and the Galaxy S8 pack a headset. If you look at it from the perspective of the Samsung marketing team, the bundled AKG earbuds that should be worth $99 on their own can actually make the S8 cheaper than the OP5, though that's not strictly how it works.
Availability and price winner: OnePlus 5, but only just. OP5 is the cheapest outright, but a lack of headphones in the box, regional price differences and promo offers on the Galaxy S8 make the 5 less of a deal than OnePlus is typically known for. Plus it's not available in all markets. The iPhone 7 Plus is easily the most expensive, especially outside the US.
The iPhone 7 Plus' design is a bit long in the tooth already - it is, after all, the third iteration of basically the same design. Sure, subtler antenna bands, non-clickable Home button, dual camera - the slight difference are there, but it was by no means a fresh-looking phone when it launched 10 months ago, much less now.
The iPhone's often been criticized for its sizable bezels and that applies to this 7 Plus too - it's one of the largest handsets with a 5.5-inch display. It's quite heavy too - 33g more than the Galaxy S8 and 35g more than the OP5.
Nonetheless, OnePlus deemed the 7 Plus worthy of taking some design cues from. Yes, some call it blatant plagiarism. There's not much along the lines of continuity from older OnePluses when it comes to camera placement - if OP had put the camera along the phone's central axis, the copycat accusations would have been put to rest (okay, maybe).
However, the OP5 does have a personality of its own - the so-called Horizon Line along its entire perimeter and the tapered back towards the edges make it quite the different device to look at, but even more so in your hand. You might have to explain to the uninitiated why your iPhone has no Apple on its back, but with more publicity that will quickly stop being an issue.
The Galaxy S8, on the other hand, isn't really like any other. It does, indeed, have a dual-curved display, and Samsung's had a few of those in the past. But the S8 in particular frankly looks a generation or two ahead of the competition when it comes to style. And its impressively posh appearance doesn't come at the expense of compromised handling like previous 'edge'-s did.
All three phones have a mute switch on the left side. Oh, wait, that's the Bixby shortcut on the S8 - a hardware button to access Samsung's personal assistant. The iPhone's switch toggles between ring and mute, while the OnePlus adds a 'do not disturb' position in between.
The OP5 and iPhone 7 Plus have front-mounted fingerprint sensors - embedded in the Home button. Although, technically, that was never a button on OnePlus phones - instead it was a touch-sensitive area since day one. Or rather since OnePlus 2. Apple only made it that way for the iPhone 7 generation - now, who's copying who? Apple's Taptic engine does nicely serve its purpose of providing localized feedback mimicking the press of a button - something nether OP, nor Samsung can claim.
Speaking of home buttons that aren't really buttons, Samsung lost its signature hardware Home key too in the S8 generation - there was simply no room for it. It's been replaced with a pressure sensitive area where the button used to be, which is always-on, so to speak - you can get to the homescreen by pressing it regardless of what's being shown on the screen.
That's all very nice, except the S8's fingerprint sensor had to go to the back of the phone next to the camera. That means it's generally hard to hit it by feel only, and also you're likely to be smudging your camera lens. The iris scanner replaces it reasonably well for unlocking, though we've had mixed success with it - some are fine with it, others hate it.
The Galaxy S8 is the only glass-backed phone here and that actually makes it grippier than the smooth aluminum iPhone 7 Plus and OnePlus 5. Add to that the fact that the S8 is the most compact of the three, and it becomes clear which is the most comfortable phone to handle.
Design winner: Samsung Galaxy S8. It's the most pocket-friendly phone for its screen area, handles well, and looks ahead of its time - it's enough to offset the awkward fingerprint reader placement. The OnePlus 5 takes after the iPhone 7 Plus but is more compact and has a personality of its own. The iPhone 7 Plus looks outdated, has huge bezels, and weighs a lot.