@MissyR Campy do an 11sp triple, if money isn't an issue, The RD has to cope with two things range (primarily can it do a large rear gear i.e. 30 or 32th) and capacity, this is how many teeth it can accommodate front and rear, generally a GS (medium) rear cage is needed for a triple and a compact double sporting a 50/34, though not always, depends on the range of the cassette. Not dure if your front is 30/42/52 or 30/39/50 taking the former as I suspect it might be, the capacity is the difference between the large and small rings in this case 22th, then take a cassette of say 12-28 this has a capacity of 16th add these two together you get 38th so the RD has to cope with this amount, another example take a pretty normal road bike 52/39 and a 12-23 this needs 13+11=24 so a short cage is needed (SS) finally a compact double of say 50/34 and 12-28 16+16=32 a short cage should be ok in this set up, however if you fit a 32th rear on the same you would need a GS.
I am not sure but I think the Tiagra 9sp chainset uses a 74/130 BCD in which case you cant fit a smaller inner ring say a 28th ( I had a set up with a 26/38/50 front, so it should work on a 30/42/52) and say a 40 middle, I have also had a 30/40/52 setup and know that works)
Changing just the front inner ring alone from 30 to 28 on a 25th rear will give you on a 700x25c wheel an extra two inch, 31.8 to 29.7" on a 50/34 you would need to have a 34x32 to have a lower gear even a 10sp tiagra will have big gaps, 12-13-14-15-17-19-21-24-27-30, the 24-28 are not to bad, but the 15-17-19 those missing you really notice them, sticking with a 9sp triple of say 28/40/52, and a 10sp double compact (50/34) to have the same range of about 29" to 110"
This is a chart showing the gear inches if you just changed the middle and the inner
This on a 50/34 12-30 rear 10sp
Its easy enough to make your ratios by splitting the cassette if you wish (with shimano you grind the rivet head off and separate the cogs though the spacers are quite delicate on 9sp's) and you could start from 13th from one cassette which is not so bad on a 52x13=105"
There are so many variations you can do. The 50/38/26 I had worked well on my 12-13-14-15-16-17-19-21-24-27 but not so well on my 12-23, too big a gap dropping down to the inner 26th, had to go back to many gears to be in the same range (50/34 chainsets suffer this) You could fit a 24/36/48 front and 12-25 to give
Though not sure on the 24 inner, but this is what I had and it worked pretty well, its just i found I was never using the lower gears.