Yes, currently all FSC carriers which operates international flights (with exception of Lion Air Group, AirAsia Group, Sriwijaya Air and Nam Air) operates from Terminal 3.
Nice to hear that you’re having a great experience flying in this sector. I also have tried A350 on this route but vice versa (Singapore-Jakarta) and at that time, SIA was operating at Terminal 2 instead of Terminal 3.
And like @Raidon23 previously stated, if you are staying in Singapore for awhile during your transit (with minimum of 3 hours), try out the Jewel. You can find many interesting restaurants (Like Shake Shack, the famous Burger and Lobster 😍), interesting playgrounds in the terrace floor (Like Mirror Maze, a Maze, Discovery Slide, and also Sky Nets!) and most importantly, the amazing rain vortex! It’s just an amazing experience to the point that 3 hours may not be enough 😂
Looking forward for photos from Singapore-Frankfurt sector 😊