A femtosecond laser micromachining technique is used to write optical waveguides directly into the two sides of a three-core fiber from a single-mode fiber. The intermediate fiber core is welded and aligned, with an optical waveguide length of $1420~\mu$ m and an angle of 1.15°. This method realizes parallel writing technology for multicore fibers, achieving a fiber Bragg grating (FBG) signal-to-noise ratio (SNR) of 20 dB. The temperature and strain characteristics of the sensor are studied. Through experiments, the average temperature sensitivity is found to be 11.3 pm/°C, and the strain sensitivity is 1.69 pm/ $\mu \varepsilon$ . The sensor exhibits good temperature and strain response characteristics and stability. This method provides a novel approach to parallel writing in multicore fibers and lays a foundation for subsequent experiments involving curvature and torsion characteristics. It holds great potential for applications in ocean temperature and depth measurement.