Load of 70 mA at 170 V output is a 20-watt design. The area of SMPS converters design is somewhat sophisticated and requires some skills. For this range, especially with so low input voltage, the design will have challenges, and would require more than a transistor and diode, but a more advanced controller IC.
I would strongly advise to look into industrial offering of boost circuits, say, starting at Texas Instruments SELECTOR. Plug in your initial numbers, 3 Vin, 170 Vout at 0.07A, and you will have a selection of controllers that will be capable to get these parameters. The list will show only two basic parts, LM3478 and LM3488. Which already indicates a serious challenge.
Then try to use their automated DESIGNER to generate the entire design, which will offer a suitable selection of all components.
The result is unfortunate for you:
We're sorry, we could not find a solution to match these requirements
for the following reason(s): Can't find suitable FET for the design.
Try reducing load current
Attempt of use more general TI WebBench Designer evaluates maybe a hundred of controllers, but with the same result - can't do this. If TI can't find suitable FET for the design, there are little chances that you can.
I would say that you need to reconsider your design parameters, likely to use higher Vin, or more realistically evaluate the load current, or use a transformer-based converter topology with plain output rectifier.