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In general, yes, QoS does work.
Will it work in your scenario? Don't know, as you have not posted QoS configurations, nor described QoS expectations/requirements, nor described you actual traffic flows/usage.
With the information you have provided, one potential issue is a 3845 can struggle, performance wise, with 100 Mbps. Assuming though the the 3845 is the 1st major bottleneck (i.e. the 100 Mbps link) fair-queue, alone, configured on that interface might provide about 90% of the benefit of what you can obtain from QoS.
Without getting into an indepth analysis of QoS for your situation (and my consultation billing rates are reasonable ), QoS basically manages constrained bandwidth. It often works extremely well when you need to guarantee low resource usage demanding flows, e.g. VoIP, not be adversely impacted by concurrent high resource usage demanding flows, e.g. FTP copies of large files. Properly done, it can even sometimes improve effective performance of high resource usage demanding flows.
