The arrows outside the grid indicate that the nearest three digits in the corresponding direction are in ascending or descending order (the highest number is always in the direction of the arrow). All possible arrows are given, so if there is no arrow, the first three digits do not form an increasing sequence in either direction.
Here is my solution:
None of the inward lines-of-three around the edges have an increasing or
decreasing sequence except where marked by arrows showing their direction.
The question tags allow a computer solution, so I built on my answer to
Knights in a Complete Sudoku Board which used C code to solve it.
As with the Knights problem, I added additional rules.
I set out to solve it by hand, on the basis of runs-of-three being consecutive.
If so, in diagram B some of the runs can be filled immediately.
In diagram C the only possible sequence for the second arrow across is $6 \ 7 \ 8$.
But it is obvious there is nowhere to place a $3$ in that corner box.
So in view of a comment under the question and no hand-made solution posted within
several hours I worked out this answer. Some may say it's not very logical and I
only have to dial it into a program but that simply isn't true. It's just a different
logical process, and it took me longer than it usually does to solve a Sudoku by hand.
I did not find any other solution.