Why do you need to get it down to one large partition? It should be easy to combine the last 2 partitions into a single partition but you will need to jump thru a few hoops to eliminate that first 100MB partition.
You should be able to right-click on the partition D: partition and select "Extend Volume" which will allow you to extend the D: partition into the unallocated space to the right. Note: Disk Management can only Extend into space to the right of a partition, if you want to extend to a partition to the left, you'll need a 3rd party program to do that.
Is this a data drive or a boot drive? If it's data then you can delete the 100MB 1st partition to make it unallocated but in order to add it to the D: partition, you will need a 3rd party partitioning program such as Best Free Partition Manager for Windows | MiniTool Partition Free