I used to do 3x per week routines. Not saying this is the best but it worked for me for that time. Keep in mind I stay away from shoulder workouts for health reasons.
During this time I would do A/B/A2/B2. This would be chest/back, legs with mild arm work, chest/back, legs. This gives you a good frequency of working opposing muscles together (back with chest and quads with hamstrings...etc). You could also do a push/pull. This would also give you a different routine each week since it’s rotating 4 workout days but you only do them 3x a week.