Wash all cucumbers, bell peppers, and onions thoroughly under cold running water.

Finely chop the cucumbers, bell peppers, and onions into small, even pieces.

In a medium pot, combine the chopped vegetables, granulated sugar, apple cider vinegar, mustard seeds, celery seeds, turmeric powder, and salt.

Bring the mixture to a boil over medium heat, stirring occasionally until the sugar dissolves.

Reduce heat to low and simmer for 30 minutes, stirring occasionally, until the relish thickens.

Carefully ladle the hot relish into sterilized canning jars, leaving 1/2 inch of headspace at the top.

Wipe the rims of the jars to ensure a clean seal, then place the lids and screw on the rings until snug.

Process the jars in a boiling water bath for 10 minutes for shelf-stable preservation.