As a fellow UWA direct pathway med-sci first year, as much as I'd like to say that it'll all be OK, 3 Ds and one P would leave you on a 5.5 GPA exactly right now. Considering the med sci units get much harder next year from what I've heard (from second years and from a conversation with Rob White as well), this is concerning given that you need a 5.5 GPA minumum at the end of third year to proceed to postgrad med.
Having said that, to get a direct pathway is quite difficult so I'm sure you have the capacity to keep up the average. And it's not like the rest of us are finding the units easy - there was a LOT to remember for that IMED1003 test (and don't even get me started on vitamins!!).
Remember that med sci is only half your units so it's only half the battle. If you're planning on doing a second major, maybe this is not for you - try and choose really easy electives and broadenings to boost your GPA and allow you to focus on med sci.
Most importantly, you should absolutely talk to the health and medical sciences student office now so that you can get the help you need to maintain that 5.5 GPA before things get really tough next year.
All the best fellow soldier