I am almost positive you will likely not stop at three pieces. There are too many lovely designs to do that!
I do not think you are being irresponsible. I think the fact you are asking these questions means you are aware that financial stability should always play a factor for the choices one makes in life. Each one of our stories are different in every aspect: education, family structure, financial position, preferences and values/markers for what is important to us. I also believe financial awareness/education is important and I wish it was a course every student had to be taught in school!
If you are making decisions which will set yourself up for a solid future, then what you do after that is really just personal choices/preference. If your preference is luxury goods, then so be it. Some choose to buy material goods; others choose to travel, others donate and volunteer; others do a mix of these things. Some people squirrel it all away for the long, hard winter and then never make it there; others do. It is important we have a degree of financial independence which, incase life hands you a hard card, will allow you to get back on your (and your kids'- if appl) feet. ALWAYS cover these bases first. Build your cushions: as a family, a mother (if applicable) and as an independent. After that, if your husband and you are on the same page; then world is your oyster (so to speak).
To answer your question: I started in my late twenties with jewelry, including VCA (of which some of my tastes have changed). I toggled between bags, jewelry and RTW during my 30's and then now in my 40s I feel I am at a point where I am filling gaps in what suits my fancy. I certainly feel less "rushed" and feel somewhat concerned about what SM is doing to the young generations. These material items do not mean one is a better person because of them but sadly there seems to be this checklist that needs to be ticked off. Personal choice and identity also seems to be lost-everyone is on the same bandwagon and the horses seem to change paths every couple years.
Shopping is fun, but nothing compares to the experiences we have with loved ones and the world. There is so much to see, do, read and learn. As you move throughout life, your tastes will likely change so what you think is important now may not be what you think is important at 50.
I kind of went on a novel rant but a few things have come to mind from this discussion. Anyway - my opinion is: Cushion first! They sky is the limit from there. Have fun.