This is an example of trochaic trimeter that employs catalexis, so that the last syllable is cut off:  / * | / * | / (*). As with many poems by William Blake, the meter is highly rhythmic but not mechanical.

                              /         *    |  /     * |    /      (*)
                            Why should joys be sweet
                            Used with deceit,     ["used" is pronounced with two syllables]
                            Nor with sorrows meet?

                            But an honest joy
                            Does itself destroy
                            For a harlot coy.

Here's another by John Keats:

                             /     *  |  /   *  |   /      (*)
                          HITHER hither, love---
                                  'Tis a shady mead---
                            Hither, hither, love!
                                  Let us feed and feed!

                            Hither, hither, sweet---
                                  'Tis a cowslip bed---
                            Hither, hither, sweet!
                                  'Tis with dew bespread!

"The Flower" by Tennyson is even more ambiguous; it could possibly be considered as iambic trimeter with a lot of headless lines, but there are enough lines that begin with a stressed syllable to consider it trochaic trimeter. In that case, though, there are some lines to which an initial syllable has been added; this is anacrusis.

                         Once in a golden hour
                                 I cast to earth a seed.
                            Up there came a flower,
                                 The people said, a weed.

                            To and fro they went
                                 Thro' my garden bower,
                            And muttering discontent
                                 Cursed me and my flower.

                            Then it grew so tall
                                 It wore a crown of light,
                            But thieves from o'er the wall
                                 Stole the seed by night.

                            Sow'd it far and wide
                                 By every town and tower,
                            Till all the people cried,
                                 "Splendid is the flower!"
 

These lines by Henry Constable seem best interpreted as (partly) catalectic trochaic trimeter:

            Venus fair did ride,
                      Silver doves they drew her  [perfect trochaic trimeter: / * | * / | / *]
            By the pleasant lawns,
                      Ere the sun did rise;
            Vesta's beauty rich
                      Opened wide to view her,
            Philomel records
                      Pleasing harmonies;
                  Every bird of spring
                      Cheerfully did sing,