Hi @gandaio1 ,
I know how it feel, bro, co'z I've got similar experience with frustration before on some ThinkVision + ThinkPad. Here's something from WiKi that may help in your case while it DID solve mine!
HD 2500 and HD 4000 GPUs in Ivy Bridge CPUs are advertised as supporting three active monitors, but many users have found that this does not work for them due to the chipsets only supporting two active monitors in many common configurations. The reason for this is that the chipsets only include two phase-locked loops (PLLs); a PLL generates a pixel clock at a certain frequency which is used to sync the timings of data being transferred between the GPU and displays.
Therefore, three simultaneously active monitors can only be achieved by a hardware configuration that requires only two unique pixel clocks, such as:
Despite your Dell monitor on VGA, let's check out the 2 Lenovo ThinkVision:
L2251x (page #23)
Voilà! You got it now, right? Yes! The 2 different ThinkVision occupied 2 unique pixel clocks already and thus no rooms for the third connection (VGA). It also explained why 2 displays in your case always works no matter if DP#1+VGA or DP#2+VGA or DP#1+DP#2.
Just a short test: Try connect 2 identical ThinkVison to the 2 DP ports and the Dell monitor on VGA, then see if it do the tricks. Good luck, dude!