@srv4ever 

Short answer, I dont think this is possible easily.  The 3x connections are going to have different IP addresses as they are seperate lines and not a lot you can do about that. 

The only thing i can think of is a cloud gateway in Azure with bastion and a gateway server, you have all 3 connections VPNed or tunnelled into Azure where its hitting the gateway, which is pointing to one external IP provided by Microsoft.   All traffic goes to that Azure IP which balances down to the 3x connections you have, therefore externally you have one IP

It would be messy to setup and stupidly costly in terms of azure licence..    at a guess you are talking $1000 per month for the bastion and gateway based on 500Gb traffic 

@Philbert 

Holy Cow!
That is a lot of work and money.
I hope there's a simpler and more affordable solution to that.

Thank you for your response!

Doing some research I'm wondering if I could connect those ISPs and use a VPN service (like ExpressVPN) and tunnel all my traffic through it. (OpenVPN or even L2TP/IPSec)
Could an RT480T+ handle it? Or should I buy another solution?

@srv4ever 

Ask google for "openmptcprouter". 

I am currently running 2 ISP with that one, at the moment on an old Laptop with 2 additional usb2eth ports and a vultr VPS. 

It works amazingly well, I'm very happy that someone gave me that hint. Can be a very cheap solution (ca. 5$/€ p. month for VPS + 50-500+€ for hardware like Raspberri Pi, Mini PC or even a Protectli).

One ISP ist pretty fast but has sometimes short outages of a few seconds. The other one is stable but not as fast as the first one. 

Openmptcprouter aggregates both (it's really nearly both speeds just added), the ping is in the 40s to 50s. I don't even notice when the fast ISP is off if I don't look at the speed, the internet stays seamlessly alive. 

My employer's VPN is also working without losing connection. 

@oettinger31 
Thank you for replying!

I'm going to test it this weekend to see if it works.