Designing a microstrip patch antenna at 3GHz frequency is very interesting task to the researchers due to wide applicability for IoT, body wearable antenna, WLAN, ISM communication etc. In this paper, a circular microstrip patch antenna with resonance at a frequency of 3GHz has been designed using FR4 substrate and inset feeding using CST studio. Initially, the antenna is manually tuned at the 3GHz operating frequency and obtain S11 is -38dB which is best compare to the existing works at this frequency. Next, the parametric influences have been observed by varying several antenna design parameters (i.e. patch radius and substrate height) and observing the respective variation in output like S11, bandwidth. In the next phase of this work, surface fitting technique has been used to model the 3GHz circular patch antenna that helps to predict the output S11, resonance frequency and bandwidth with satisfactory prediction accuracy. Surface-fitting model will help to reduce the effort and time required for redesigning and simulation of this type of circular patch antenna in future.