Is it necessary having third party come to certify accuracy of my equipment periodically

You asked

do I have to have a third party come in a certify the accuracy of my equipment periodically?  

The requirement is to ensure the correct capability and performance of you equipment and to document the calibrations for all equipment that contribute to the overall measurement uncertainty of test or calibration results .This means having a linked unbroken calibration chain and known measurement uncertainty (MU). Then on an ongoing basis maintain confidence  (clause 6.4.10) by either performing your own intermediate checks or perhaps own calibrations. Whether you can do you own calibrations all depends on the type of equipment, knowledge of standards and specifications required, and the purpose. If you require measurement uncertainty (MU) values to use in you uncertainty calculation then an external ISO 17025 accredited laboratory can provide that, giving you the required metrological traceability, the MU and a suitable ISO 17025 calibration report. Also consider requirements from your accreditation body as they will typically have a policy on the options or approach required. 

You also asked

If so how often?"

The calibration period must be determined by you, based on accreditation body requirements, risk or best practices. Refer to ILAC (international organisation for accreditation bodies) for policies and guidelines.

Consider, for example, that the annual calibration of a weight set that is used for routine analytical balance verification may be excessive, if the personnel are trained to handle the weights properly, and they suitably stored. For your sensors and gauges, you should refer to recommendations from suppliers and best practices.

The article: What does ISO 17025:2017 require for laboratory measurement equipment and related procedures? at https://advisera.com/17025academy/blog/2019/07/25/iso-17025-measurement-requirements-of-the-standard/

The ISO 17025 toolkit document template: Equipment and Calibration Procedure at https://advisera.com/17025academy/documentation/equipment-and-calibration-procedure/

Also refer to  ILAC P10:07/2020 ILAC Policy on Metrological Traceability of Measurement Results and ILAC G24:2007 Guidelines for the determination of calibration intervals of measuring instruments, available from https://ilac.org/publications-and-resources/