Is there any advantage in implementing ISO 45001 for a design organization (tith 3 employee)?
Apart from being a top mgm. / customer requirement, why would a design organization implement ISO 45001?
TLDR: for a company of 3 people, ISO 45001 is a massive overhead unless your people are doing activities which are a threat to their safety or health.
Long answer: There are other reasons to implement ISO 45001 (and please remember that implementing a standard does NOT mean that you have to certificate to it Unless you agree to) other than your Customer requires it.
The primary reason to implement ISO 45001 would be put a structure around keeping your employees safe and healthy. You don’t say whether you have implemented or are certificated to any other standards. If, for example, you’re already certificated to ISO 9001 or ISO 27001 then lots of the core for the implementation of 45001 is already in place.
In your scenario I wouldn’t recommend you do this, and if a Customer (or potential customer) insists, ask them to justify the requirement for the work you are doing for them. Too many companies take a hard supplier line that because they (the Customer) is certified, they expect all suppliers to be too rather than doing what the standard suggests and taking a risk based approach.