Refer to the Bayonet 3 manual but I think for 30mm of spacers they specify a configuration of 20mm spacer + adjustable bridge + 5mm spacer.
Now I recall that bridge being less than 5mm thick, perhaps closer to 2mm, but it'll get you close.
If you wanted another 5mm, I think you can swap the 5mm spacer to 10mm, or add a 5mm below the bridge and keep the 5mm above it - but again, check the manual for the alloy aerobar before acting on my advice!
https://cdn.shopify.com/s/files/1/0021/5701/9254/files/Bayonet3_Alloy.pdf?v=1531528904
Now I recall that bridge being less than 5mm thick, perhaps closer to 2mm, but it'll get you close.
If you wanted another 5mm, I think you can swap the 5mm spacer to 10mm, or add a 5mm below the bridge and keep the 5mm above it - but again, check the manual for the alloy aerobar before acting on my advice!
https://cdn.shopify.com/s/files/1/0021/5701/9254/files/Bayonet3_Alloy.pdf?v=1531528904