Based on your current role as a safety supervisor with a salary of 3k and also handling the scope of work for a quantity surveyor, it is reasonable to ask for a salary increase. Considering the additional responsibilities you are taking on, a realistic and logical amount to request would be a 10-20% increase in your current salary. This would reflect the added value you are contributing to the organization. It's great that management is open to discussing a salary revision, so be prepared to present your case with evidence of your performance and the value you bring to the company. Additionally, highlight how your dual roles benefit the organization and justify the salary increase. Good luck!