We present a new type of 3D printing pen fabrication of flexible passive UHF (ultra-high frequency) RFID (radio frequency identification) platforms. The used 3D printing material is cost-effective and fully biodegradable. The tag antenna is fabricated from conductive thread. This antenna and antenna-IC interconnections are fixed inside the 3D-printed platforms with 3D-printed fasteners. Thus, our solution requires no additional process steps or chemicals. For wireless evaluation, the platforms are firstly bended around different size cylinders, with and without a 3D-printed top layer. Further, the reliability of the platforms in high moisture is also tested, by keeping the platforms in water for 1 minute. Based on these preliminary tests, our manufacturing method enables establishment of highly reliable eco-friendly platforms with excellent wireless performance. The developed solutions have versatile future applications, especially in the field of wearable electronics, where lightweight, cost-effective, flexible, and waterproof platforms are needed.