The Electrical standards for ultra-low voltage state that both the positive and negative conductors (wires) of a solar array need to have a disconnection function. (this means the ability to be turned off) When the panels are producing energy disconnecting under "load" will cause an arc as the contacts are opened. Automatically reconnecting circuit breakers can become welded in place while connecting or disconnecting under load which is why DC Isolating Switches are used, being manually operated you will know it's still working correctly as you turn the rotary on and off. However, these DC Isolators are expensive, more expensive than using a manually operated circuit breaker which you can manually trip and reconnect. This is why we supply double pole (pos and neg) circuit breakers in housing as its a 2 birds with one stone approach. Protection and Isolation.

Solar panels themselves can't short circuit even if you connect the two outputs of the panel together as the internal wiring is rated higher than the panels output. If, however, you have 3 panels in series and one fails. The power output of the other 2 panels will be more current than what a standard 16A circuit breaker would be capable of handling. For this reason, circuit protection may not be required, but isolation is. On 1 or 2 panels a single 16A 500VAC circuit breaker is adequate. But with 3 panels, you should isolate each panel with a circuit breaker to ensure safety and isolation can be achieved.

At this point, 3 circuit breakers cost more than 3 fuses and 1 x DC Isolator so the choice is yours to make. Isolate and protect yourself.