Animal Welfare Professionals

Looking for training tips to help keep 3 yrs old foster cat off countertops?

These work pretty nicely. They aren't designed to cause pain, just to startle them. https://www.chewy.com/ht-pet-snappy-trainer-pet-deterrent-2/dp/134209

For an easy "DIY deterrant," (most) cats don't like touching tin foil, so you can put tin foil on the counters for awhile (I just tape it down). Same goes for double-sided tape (though I've only used that as a sofa-scratching-deterrent!) .

They also sell these "slightly pokey mats" that are uncomfortable on their paws: https://www.chewy.com/petsafe-scatmat-electronic-pet/dp/48625

I've also heard of motion-activated water-sprayers that you can place strategically (so it'll spray him when he jumps up and scare him). 

Double sided tape works too.  Cats don't like the feeling of it under their feet.

Remember to look at what they want to do, not just the deterrent.  What are they accomplishing on the counter - a view, access to their favorite human, attention, food?  Fulfill that need in a more appealing way!  If they are food motivated, you can try playing "find it" with them at random intervals (tossing a piece of kibble, or something more high value, if needed).  Then you can play that game when they're looking to jump up on the counter.