I have a laptop with thunderbolt support. Using a 3 way splitter I'm able to connect 3 monitors. Here's some specs:

'''
$ inxi -Fxzc0
System:    Host: io Kernel: 4.14.44-1-MANJARO x86_64 bits: 64 compiler: gcc v: 8.1.0 Desktop: KDE Plasma 5.12.5 
           tk: Qt 5.10.1 Distro: Manjaro Linux 17.1.10 Hakoila 
Machine:   Type: Laptop System: Dell product: XPS 15 9560 v: N/A serial: N/A 
           Mobo: Dell model: 05FFDN v: A00 serial: N/A UEFI: Dell v: 1.6.2 date: 11/12/2017 
Battery:   ID-1: BAT0 charge: 88.1 Wh condition: 88.1/97.0 Wh (91%) model: SMP DELL GPM0365 status: Full 
CPU:       Topology: Quad Core model: Intel Core i7-7700HQ bits: 64 type: MT MCP arch: Skylake rev: 9 
           L2 cache: 6144 KiB 
           flags: lm nx pae sse sse2 sse3 sse4_1 sse4_2 ssse3 vmx bogomips: 44944 
           Speed: 3339 MHz min/max: 800/3800 MHz Core speeds (MHz): 1: 3373 2: 3400 3: 3400 4: 3400 5: 3400 
           6: 3400 7: 3400 8: 3400 
Graphics:  Card-1: Intel driver: i915 v: kernel bus ID: 00:02.0 
           Card-2: NVIDIA GP107M [GeForce GTX 1050 Mobile] driver: N/A bus ID: 01:00.0 
           Display: x11 server: X.Org 1.19.6 driver: intel unloaded: fbdev,modesetting,vesa 
           resolution: 3840x2160~60Hz, 3840x2160~60Hz 
           OpenGL: renderer: Mesa DRI Intel HD Graphics 630 (Kaby Lake GT2) v: 4.5 Mesa 18.0.4 
           direct render: Yes 
Audio:     Card-1: Intel CM238 HD Audio driver: snd_hda_intel v: kernel bus ID: 00:1f.3 
           Sound Server: ALSA v: k4.14.44-1-MANJARO 
Network:   Card-1: Qualcomm Atheros QCA6174 802.11ac Wireless Network Adapter driver: ath10k_pci v: kernel 
           bus ID: 02:00.0 
           IF: wlp2s0 state: up mac: <filter> 
           Card-2: Qualcomm Atheros type: USB driver: btusb bus ID: 1:2 
           IF-ID-1: br-abd3c2b482b1 state: down mac: <filter> 
           IF-ID-2: br-b7b4ade7beae state: down mac: <filter> 
           IF-ID-3: br-ca990f09811e state: down mac: <filter> 
           IF-ID-4: docker0 state: down mac: <filter> 
Drives:    HDD Total Size: 592.63 GiB used: 179.60 GiB (30.3%) 
           ID-1: /dev/nvme0n1 model: KXG50ZNV512G NVMe TOSHIBA 512GB size: 476.94 GiB 
           ID-2: /dev/sda type: USB vendor: SanDisk model: Ultra Fit size: 115.69 GiB 
Partition: ID-1: / size: 451.35 GiB used: 135.05 GiB (29.9%) fs: ext4 dev: /dev/dm-0 
           ID-2: swap-1 size: 17.07 GiB used: 57.6 MiB (0.3%) fs: swap dev: /dev/dm-1 
Sensors:   System Temperatures: cpu: 66.0 C mobo: 50.0 C 
           Fan Speeds (RPM): cpu: 3220 
Info:      Processes: 222 Uptime: 22h 49m Memory: 15.52 GiB used: 4.58 GiB (29.5%) Init: systemd Compilers: 
           gcc: 8.1.0 Shell: bash v: 4.4.19 inxi: 3.0.08
'''

I'm using only intel to render (I dissabled the nvidia card for now).

KWin using any OpenGL backend just garbles and mirrors the output on all three monitors. Here's a bit more info you might find useful https://forum.manjaro.org/t/thunderbolt-3-monitor-adapter/48452/5

Please let me know what do you need to debug this problem.