With a keen interest in courses surrounding biology, chemistry is generally a favourable choice due to its complimentary aspects to biology. However, do not feel forced to take certain subjects if you don't enjoy or aren't good at them.
If you choose to take Mathematics but fail to keep up withe work load, it will seriously bring down your prospects of achieving solid A-Level grades. Due to your decision in only choosing 3 A-Levels, you will be more vulnerable as the majority of universities ask for 3 A-Levels.