I'm assuming you want to keep the option of letting them filter back to 2015? If so, then drag another one of your Open Date fields from your dimensions to your filter shelf - have this one set to year of Open Date. Keep all of the years in the filter. Right click on it and select "Add to Context". This will make sure this is processed before your other date filter.
Now on your original Open Date filter, click the right x in the top right to remove the fixed dates. And on your year filter, select only 2017, 2018, and 2019. This will make sure the original date filter expands to include new days and is also filtered to 2017 and later by default. If users want to go back to 2015 they can add in 2015 and 2016 on the year filter.
Let me know if you have any questions.