It is acceptable to connect solar panels either parallel or in series.
In parallel, the voltage will not change regardless of how many panels are connected together. Each additional panel will add amperage to the system. This would be beneficial if your usage demands more amperage but requires the lower voltage. If you wish to recharge a bank of 12 volt batteries, parallel would be best. The use of a charge controller would also be necessary.
If you have 3 panels that produce 5 amps at 12 volts each, the (parallel) output of your system will be 15 amps at 12 volts.
In series, the voltage will increase but the amperage will remain the same. Those same 3 panels would now produce 5 amps at 36 volts.
Series wiring is beneficial if you need to run wiring a long (e.g. 100 feet) distance. Smaller gauge wire can be used due to the lower amperage flowing through the wire.
If wiring in series, all of the panels should be of the same brand, size and age to maximize your efficiency. A series wired solar array is only as powerful as the weakest panel.