Merging different tag filtering with Osmosis

Your error message is explained in another answer (the merge task can merge only two pipes, so you need two merge tasks). But there's another way to address your problem, which is to avoid merging entirely: You're filtering one tag pattern in each of three parallel pipelines, then merging those pipelines. In fact, each tag-filter task can accept more than one tag pattern, and will accept/reject entities if they match any of those supplied tag patterns. Here is an alternative command creating a single pipeline that should produce exactly the same results as your three-pipeline approach:

osmosis \
 --read-pbf switzerland.osm.pbf \
 --tf accept-nodes sport=* amenity=* leisure=* \
 --tf accept-ways sport=* amenity=* leisure=* \
 --tf reject-relations \
 --write-pgsql host=128.178.1.1 database=myDB user=myUsn password=myPwd

However, both your command and the one I proposed will probably not produce the output you are expecting. Both commands are retaining all nodes and ways that have a sport, amenity, or leisure tag, but they are not retaining any nodes referenced by these ways if the referenced nodes don't also have a sport, amenity, or leisure tag. This is the purpose of the used-node task: it will retain any nodes that are referenced by a retained way.

We can't just insert a used-node task into this single pipeline, because we want to retain both the nodes with certain tags and the nodes used by ways with those tags. So here you need two pipelines, but only one merge task:

osmosis \
 --read-pbf switzerland.osm.pbf \
 --tf accept-nodes sport=* amenity=* leisure=* \
 --tf reject-ways \
 --tf reject-relations \
 --read-pbf switzerland.osm.pbf \
 --tf accept-ways sport=* amenity=* leisure=* \
 --tf reject-relations \
 --used-node \
 --merge \
 --write-pgsql host=128.178.1.1 database=myDB user=myUsn password=myPwd