I have a train ticket that goes from Liverpool to Hull and has 2 changes. It goes from Liverpool Central to Liverpool South Parkway, Liverpool South Parkway to Leeds and Leeds to Hull. I was wondering whether I can just start my journey from Liverpool South Parkway? Thanks for your time.
It depends what sort of ticket you have.
If your ticket is an "Advance" single, this means it's an inflexible ticket, and any trains for which the time is specifically printed on your ticket (or a separate reservation coupon) must be the ones you use (where a time is not printed, the rules are a little ambiguous but it's usually taken to mean you may use any train that does not offer seat reservations). In these cases, by the letter of the rules, you must complete your whole journey, so theoretically you would have to start from Liverpool Central. However, recently the Rail Delivery Group (an industry body which is an association of train operators) have been advising individual train operators to show discretion in these sorts of cases. Coupled with the fact that (as pointed out in the comments below) there's no way for the barrier staff to tell that you've just walked into the station vs having arrived on a train, I would probably be comfortable with starting late at South Parkway myself, but it's up to you whether you want to go against the letter of the rules and so take the (small) risk of having to pay for the journey again.
If your ticket, on the other hand, is an anytime single or return, or an off-peak or super off-peak single or return, you can start your journey "late" at Liverpool South Parkway, assuming the fare is for Liverpool Stations to Hull (correct me if I'm wrong on this point as it could change the answer). Since these fares are issued from Liverpool Stations, besides your options for starting "late" at South Parkway, you can also use the other three stations in central Liverpool (Moorfields, James Street, and Lime Street), which might be useful for you if your Leeds train runs from Liverpool Lime Street.
If you have an off-peak ticket you can
Break of journey is allowed on the outward portion of Off-Peak tickets unless otherwise indicated by a restriction shown against the ticket's validity code and in all cases on the return portion of Off-Peak return tickets. You may start, break and resume, or end your journey at any intermediate station along the route of travel on Off-Peak tickets unless the ticket restriction for the journey you are making does not allow it. If you intend to start, break and resume, or end your journey at any intermediate station, please call 03457 48 49 50 to check if it is available on your specific journey.
If you have an Anytime ticket
You may start, break and resume, or end your journey at any intermediate station along the route of travel.
I would have no qualms about doing that with any ticket. It is very very unlikely that a ticket inspector is going to stop you boarding. (And once on board at Liverpool South Parkway there will be no issue at all.)
As other answers say, it may not be strictly in accordance with the rules, but provided you are not doing so to gain an advantage (i.e. the price from Liverpool South Parkway is the same as from Liverpool Central) then it is going to be fine.
It is definitely not fine to get a cheap Advance ticket for a long journey in order to get on halfway along the route where only more expensive tickets are available.
But different stations within Liverpool, I wouldn't worry at all.