Besides biochemical changes that this 3 class of drugs cause in ones body I think the biggest problem could be in how they can modify behaviour. As I am not qualified to talk about pharmacology, I will give possible changes in behaviour that I experienced and that could negatively impact exercising (building muscle).
Opioids:
- if one has pain threshold increased while weightlifting then it can lead to overtraining and that can result in an exausted CNS or injury
- if one is injured taking more opioids will become more appealing and thus opioid use could escalate
- reinforcing negative habbit of not being in touch with ones body and signs/signals it sends that are very much needed for proper excercise/rest balance
- insulin disturbance that opioids cause is not in accordance with bodybuilding goals (if the goal is more muscle and lean physique)
Benzodiazepines:
- when lifting heavy weights being more prone to accidents, falls and injury would be my concern
- benzos are miorelaxants and being wobbly while lifting weights can lead to bad lifting technique
- they disturb most important phase of sleep and thus hinder recovery process
- they can cause behavioral disinhibition and make person more impulsive which can lead to bad judgement calls regarding excercise routine
Weed:
- in many cases can lead to significantly lessened motivation and focus
- weed induced increased apetite and eating suspect food at late hours can become a major obstacle to reaching set goals
Opioids and benzodiazepines affect most important systems in the body that successfull bodybuilding requires. Opioids are known to effect perception of pain which is crucial in bodybuilding, they mess up the intestines and slow up the methabolism. They may help to get you moving in the beginning but they are known to disrupt healthy sleep, fuck up insulin levels and tend to sedate people. Benzos mess with the most important inhibitory neurotransmitter (GABA) in the central nervous system by reducing neuronal excitability. That alone is in odds with muscle growth. It is in odds with any type of growth. Adding sleep disturbances, higher chance of injury and being impulsive while having false feeling that one is perfectly sober is big hinderance in bodybuilding. Both opioids and benzos are strong sedatives and if used long enough will make most people not excited or maybe even interested in such tasking endeavour as bodybuilding. Weed is also more often than not, especially in combination with opioids and benzos, type of drug that does not facilitate following through with types of behaviour that are based in discipline and strenuous physical activity. Combining these 3 drugs is contraproductive for bodybuilding in most people.
Having said all of the above your personal make up, possible health issues and true objectives, coupled with dosages and usage frequency can be factors that somewhat change this whole calculation. But depressants are in vast majority of cases directly oposed to goals that are needed to achieve to grow muscle without adding fat on top of them.
Sorry if my answer was not specific enough but I gave some insights based on my experience.
Nevertheless I truly hope that you will achieve your goals!