to be honest, before you seek help I would make absolutely sure your neighbours are telling the whole truth. Are they really barking for 3 solid hours when you leave them? They should be knackered when you come back.

Find out how long they bark for, which one of them or both and what triggers the barking. Are they anxious when you leave, are they guarding? By the type of barking they do you can understand a lot of reasoning behind. Then maybe you will be able to sort the situation out yourself and would learn invaluable knowledge about their personalities.

You can use a camera to see what they really get up to.
When you leave next, try and stick around the area and wait to see what happens. My dog never barks, but if left he may start anxiety barking, it is a desperate sound, it can end with a haul or a whimper. I normally stick around, I never walk out if he shows distress at me leaving and wait til he calms down. If he barks (I know they say don't go back in til they are quite, but it never worked for me, my dog is very nervous and panics easily), I go back in and ask for a firm quite and ask him to go back to his bed. Generally I will leave something for him like a stuffed kong to entertain himself and I always take it back when I return, so it works like his treat for being alone.

My OH, never bothers with this sequence and the dog might bark the whole duration of him being alone, more stressful for me to find him so distressed and it makes it harder next time to leave him...