Is it worth repairing a 3 year old Samsung dishwasher or should I just cut my losses?

That grinding noise before the drain failure is a classic pattern. The pump impeller ussually chews up something hard, a shard of glass, a pit, a piece of a broken rack wheel, and the grinding you heard was the death rattle. Two months later the motor seizes or the impeller cracks and the thing stops draining entirely. So the first tech was probably right about the part, he just gave you a useless answer about longevity. A drain pump is genrally not a $300 repair on most machines, the part is often under $80 if you look it up by model number, and on a lot of dishwashers it's accessible from the front bottom panel with a few screws and a hose clamp. If you're even slightly handy, watch a teardown video for your exact model and decide if that's worth 45 minutes of your time. That said, the pump isn't really your problem. Your problem is three failures in three years including a control board, which is the expensive one. Boards don't fail that early on machines with solid engineering. That's the pattern you're sensing, and you're not being paranoid. My own experience lines up with yours. I had a Samsung dishwasher that ate a pump, then the leak sensor tripped constantly, then the door latch went. I fixed the pump myself for cheap, felt smug about it, and then spent the next eighteen months throwing money at it one part at a time. Sold it for scrap and bought something boring. Should have done it after the second repair.

The math I'd use: if the repair costs more than about a third of a comparable new machine and the unit is out of warranty, replace it. You've got a known lemon history plus a $300 quote on a three year old box. That's a replace in my book, even before the next surprise hits.

And if you do buy new, whatever brand you land on, price out the extended warranty before you commit. If the warranty costs more than a few percent of the machine price, that's the store telling you something about the failure rates.

I'd push back on the extended warranty bit. Stores push those plans on everythign, not beacuse the machine is a failure risk but because the commission is fat. I've bought appliances both ways and the warranty money would've covered half a repair visit either way. Also that impeller theory sounds tidy but my last machine ground for a year before anything died, and when it did it was the heater, not the pump. Doesn't change the advice to replace here, three service calls in three years on a three year old box is a lemon no matter which part quit first.

One thing worth checking before you sink money into that pump, I've been doing appliance repair for about nine years and a surprising number of these 'lemon' dishwashers with repeat board and latch failures actually have a slow leak under the tub or a worn door gasket. Water wicks up into the electronics, corrodes connections, and you get cascading weird failures that look like separate problems. Pop the lower panel off and look for mineral streaks or rust lines around the base. If you find moisture damage, fixing the pump is just treating the symptom and the machine will keep eating parts.

One thing I'd do before paying anyone anything is pull the fault history, since that control board was replaced recently most of these machines keep a code log you can cycle thru on the display, and the tech's manual for your model lists what each code means. When mine did something similar the stored codes told me the pump had been erroring for weeks before it quit, which basically settled the fix vs. replace argument on its own. Also keep every receipt and service invoice together, because if a fourth thing fails inside the original warranty window on that new board you may have more leverage than you think.

We went through this with our Samsung washer a couple years back. Kept patching it because each individual fix seemed reasonable, then added it all up one day and realized we'd spent close to what a new one cost. What stings is we sold it when we moved and it ran fine for the next owners, go figure. Makes me wonder if some of these failures are just bad production batches rather than the whole brand being junk. Either way, I'd get a second quote before deciding, repair quotes can swing wild depending on who shows up.

Update: tried pulling the fault history like the tech here suggested and it actually logged a couple of drainage codes, so something real showed up for once. Also spotted some wear on the door seal while I was poking around, wich matches what the nine year guy said about water getting where it shouldnt. Gasket is on order. Question though, if the codes keep pointing at drainage after the seal swap, does that pretty much settle the fix vs replace debate?