The answer to this question depends on the laws and regulations in your specific location. Laws regarding age of consent and age gaps between individuals vary from country to country and even within different regions or states.
In many places, there are legal age of consent laws that dictate the age at which an individual can legally engage in sexual activities. If you are below the age of consent in your area and engaging in a sexual relationship with an 18-year-old, it could be considered illegal and may have serious legal consequences for the older individual, even if the relationship is consensual.
Additionally, there may be social and ethical considerations to take into account. An age difference of three years may not seem significant later in life, but during teenage years, there can be considerable differences in maturity, life experiences, and perspectives between a 15-year-old and an 18-year-old.
It's crucial to understand the legal implications and potential risks involved in such a relationship. If you have concerns or doubts about the relationship, consider talking to a trusted adult or seeking guidance from a counselor or mentor. They can provide you with support and advice to make informed decisions about your relationships and well-being.