A VFR GA pilot reported pilot a NMAC with traffic underneath when they descended slightly for traffic that was converging and above them. ATC had advised the reporter the lower traffic was at their 3 o'clock position.

Narrative

I was initially given two traffic advisories on VFR flight following from ZZZ departure. After given the two advisories ATC instructed to descend and maintain 4000 ft. as one traffic was at 4500 ft. and the other at 3500 ft. I was then able to spot the first traffic off my twelve o'clock. After this I was told to contact ZZZ1 and let them know about the two traffic advisories I was given. When checking in with ZZZ1 I told them I was at 3800 ft. with traffic off my 12 o'clock in sight but was still looking for the 3 o'clock traffic. ZZZ1 told me that the 3 o'clock traffic was right below me at 3400 ft. Immediately I started to search for the traffic as it was close and hard to spot being on my opposite side. I eventually saw it and reported them in sight while trying to climb in order to maintain proper separation. There was never a chance for collision from my perception; however; 500 ft. of separation was not maintained from my perspective.In retrospect; I feel multiple things contributed to this. Firstly; the traffic I initially spotted was high therefore when I spotted it I decided to descend to ensure separation from that aircraft which became no factor. However; very shortly after the next traffic approached me and I was 200 ft. lower than I would have liked. My autopilot was not fully reliable on all modes; therefore; I decided not to use it. I could have done a better job overall ensuring more precise altitude holds. However; had the autopilot been operational on all modes I would have used it giving me more precise altitude. Secondly; neither one of the traffic was communicating with ATC which made it hard to know what there intentions were. Lastly; I feel changing controllers while looking for traffic diverted my attention which allowed for altitude variation. However; as Pilot in Command (PIC) I could have done a better job ensuring to do this myself. As I am the one at the controls. I also feel I could have better voiced my concerns with ATC about fitting between the two aircraft.In the future I plan to fly with ADS-B in to give me better situational awareness and help keep me better informed about arising situations like this. Also; I am going to evaluate my traffic avoidance procedures with a CFI and educate myself to instill better traffic avoidance tactics.