Maybe I am out of line here, but what you are asking is dumb.

Diplexers / Duplexers - what ever you want to call them are by nature only about 50% efficient.
This means that you are throwing away half of your transmitted power and half of your capiable receive in the diplexer.. DUMB!

Antenna manufacturers makes multiple tri band antenna's.
But in order to do it efficiently - you have to start with a resonant antenna.

Because 70 cm and 1.25 CM are not multiples of wavelengths of 2 meters - the best you can come up with is a compromise antenna...

The Diamond V2000 is a good example of this.
It does 2 meters, 70 cm and also 6 meters...

They do this by making the antenna into segments and placing capacitors between the sections. The capacitors divides the antenna into segments.

The RF looks down the array, till it finds the length it is looking for and then it uses it, much like a Yagi Uda antenna for television...

The Diamond X 510 is another example - it uses 3 segments and has excellent gain.... Problem is when you make a 2 meter antenna 18' long, you end up with a top heavy vertical that snaps off when it is placed in too much wind or when it gets old and brittle.

The antenna you are looking for was already designed a long time ago.
It is called a Discone antenna...

You make the vertical whip one length, you place at least two ground radials underneath of a resonant length and put the radial system directly under the antenna.

Diamond makes a excellent example.
No need to home brew a antenna in this day and age, except for experimental purposes.

I get disgusted when I listen to the 2 meters and a couple of noob's comes on and starts telling me how they are hams because they build a J Pole antenna and are using walkie talkies and they are able to get into the one local repeater and can talk to people because the repeater is part of a linked repeater system - VOIP.....

If it was not for that repeater being linked, they would have no one to talk to, and when the 10 years comes up for the license, their call sign would be beside all the others from their VE test session that did not advance to General or higher, or did not put their license to good use while they held it.