Risk avoidance means avoiding the risk in the place in the project plan. For example, if the risk consists of relying on a supplier that usually doesn't deliver on time, then switching to a more reliable supplier will be considered as risk avoidance. You're not eliminating the risk, you're just avoiding it.
Risk transfer means that throwing the risk on a third party - for example, if the risk consists of a server failing, then signing a deal with a third party company to manage the server according to a 100% uptime SLA on the server is risk transfer. The third party company must now ensure that the server doesn't fail, or else they will have to pay money according to the contract.
Risk acceptance is just about accepting that the risk has materialized and there's nothing to be done to prevent - at this point the project manager will deal with the consequences of the risk.