Fitting Shimano pedals into a 3/4" crank socket

I've never heard of a 3/4" thread on a pedal on a bicycle. It sounds like they intend for you to buy pedals from the maker of your exercise bike.

However if you are handy, it may be possible to make a pair of adapter bushings from metal.

• The pedal threads will be 9/16" with 20 TPI (which is not the more common 18TPI) Sadly 20TPI is unique to bicycle pedals, so you're not going to be able to use plumbing parts for this.
• One has to be left-hand thread for the left pedal's shaft
• And the outside will have to match the 3/4" and whatever thread pitch is in your bike's cranks. This may be 16 TPI for UNF thread, or 10 TPI for UNC, or something weirder.
• Also its not guaranteed that your 3/4" threads are opposite-handed.

Given your bike is Chinese, its probably metric for the most part, so there's a chance your pedals have an M18 or an M20 thread, 3/4" is about 19mm so right in the middle of those options. None of them are pedal threads I've heard of though.

This would be a simple job for an engineering firm to fab up from a couple of small disks of steel or maybe aluminium. It would help if you could give them both cranks and both pedals for test-fitting too.

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There was a short lived thread of 1 inch, called Shimano Dyna drive. Adapters from that down to 9/16" look like this. Notice the inner is completely within the outer, because there's enough metal to make this possible.

The other option is to stagger them a little as in this 9/16 to 1/2 inch adapter set. Downside of these is they increase your "q-factor" which means your pedals are further apart.

Images are for description of the adaptor required, not a specific product recommendation. Pictured products have the wrong threads for OP's need.